A couple of months before emigrating I bought an Epiphone Dot 335, and began learning, then left it behind. About a year ago I picked up an acoustic, and continued learning. I have since collected my Epiphone Dot and I'm looking to change the strings on both.
 
The acoustic strings are quite dis-coloured (around the head end of the neck mainly, from open cord shapes) and it seems my naturally oily hands are not good for them over time. I tend to play folky picking stuff and occasionally strum. I think the guage is fine for me, son I'm just looking to replace with the same gauge, I'm guessing 11's.
 
The Epiphone are not dis-coloured, and there seems nothing available on regards the gauge on their website. Reading some forums the consensus seems to be they are 10's on more recent models. Which is how I'm guessing the acoustics have 11's as they seem a little thicker.  Some suggestions have be made about changing the stock strings to help with the tone. I've read some comments about it being muddy, and although I concure with these comments, I really don't have the experience to do so. So I am thinking of upgrading these, and because I'm used to the 11's on the acoustic, making these 11's too, and hoping I'll enjoy the tonal benefit with it. Again, folky picking and chord strumming.
 
Although there isn't really a question in there really, more thinking aloud so anyone wiser can chip in an say 'go for it?' or 'have you considered......?'. Note, I am in Da Nang, a city half way between Ho Chi Minh (Sai Gon) and Ha Noi, so when I find a good stockist of strings, I will probably be limited in choice and in the ability to communicate with the staff.
 
Here is a question though, should I replace one string at a time, to save the guitar from straightening then bending back (though I think it's the other way round, but you get me), or just whack 'em
  all off, so it's easier to clean and put them all back again?
 
Also the selector pot on the dot seems to have failed in the middle position and only picking up on pickup, and not both. But it works when selecting one or the other, just not both, which is annoying a for a guitar that has spent the best part of 2 years of its 2 and half years life in a hard case. So I need to find some contact cleaner and hope that's sorts it.

I have electric guitars (though am a keyboard player), and I change strings one at a time, due to the tremolos in the guitars.
 
When I used to have a classical guitar, I also used to change one at a time, to keep a more even tension on the neck, but I cannot TELL you that is the way to go - it was how I was shown by my guitar Yoda many decades back.
 
I would make sure you wash your hands just prior to playing or changing strings, and I would also suggest wiping down the strings and fret board immediately after finishing playing, to reduce oil presence on the strings.  It will help them last longer, look nicer, and play nicer, than oily ones.
 
Bob Bone
 

changing strings one at a time reduces twist pressure. on the neck and reduces the amount of bend the neck goes through. When I want to clean the fretboard I remove all.

Thanks for the replies guys, yeah I think I'll change one at a time, and just clean the space left as I remove each one. I saw something a while back about changing one at a time, but then a video today with the same guitar as mine, and he whipped them all off.
 
I think I'll just take one thick E string off and take it into town to check the gauge.
 
I have done some work looking for guitar strings, and so far in this city I have just one choice for the electric, D'Addario ProSteels. For the Acoustic, Martin 80/20 Bronze, and D'Addario 85-15  American Bronze (which don't even seem to be listed on their website now) and D'addario 80/20 Bronze.
 

When you do change all at once, alternate removing and adding low and high strings to reduce torque .

Cool, that makes sense, I get the concern now. Think I'll just do one at a time as nothing is too dirty, and it's my first attempt.
 

I owned a music store for 15 years, I must have changed 1,000 of sets in that time. I'd charged $20 for a cleaning , basic set up and included the strings. 
 
My go to brands were DAddario and GHS, I also like the new Elixir Brand. ( Martin are not my pick at all they break right away) 
 
I always remove all the strings to clean the fretboard and dust, That is a myth about the neck twisting , only a POS guitar would cause you grief by taking all the strings off, Sure if you left them off for a month! 
If its a Rosewood fretboard I use pure mineral oil to re fresh it. Wood will dry out, keep it oiled. 
Maple necks are normally finished so only need cleaning, I use name brand Guitar polishes. Right now I think it's Daddario. But good old Pledge will suffice. Really hard to mess up most finishes as they are mostly Poly urethane which is pretty indestructible. 
 
The other myth is about new strings going out of tune because they stretch>? One would hope not. 
They go out of tune because you did a sloppy job of putting them on and they are re seating in the bridge or the tuners.  
Hopefully there are now videos that show the correct way to seat an acoustic guitar sting in the bridge.
The main thing is to pull up and twist it as you push the pin in and make sure all the wrapping comes up and out. The ball should seat just under the wood of the bridge, not on the end of the pin. You'll get it if you play around and pay attention. It will feel right when done properly.  
 
Electrics are the same, Make sure you seat the ball end so it is solid and has no play. Fender Strat type bridges the ball can get jammed up in there and I would use the Fender Bullets on those bridges. 
 
I replace the strings one at a time starting with the Low E. 
Pull it from the bridge and keep tension on it while feeding it through the tuning machine.
Pull it all the way through until there is no slack.
Now pull back gripping with your right hand in the middle of the neck to gain only enough so the string is about 3" above the neck. This should result in a little more than 1 wrap on the low E and up to 3 wraps on the High E. 
If you have locking machine heads all the better. I still like to have a few wraps even on those. 
Now keep the tension on the string with your right hand as you start to wind it on ,use a string winder if you got one. Make sure the winding's are tight and going downwards. This is important to wind the string with as much tension as possible. As it winds on guide it to the bridge slot if any and the Nut slot. Keep that tension on it. repeat for all strings. 
Over tune by a 1/2 step, lower 1 step, tune to pitch. Bet you it stays in tune with only a few minor winding and bridge seating issues to deal with. 
 
String gauge:  I use Daddario. 
Lighter strings are a little easier to play on electrics , mostly for string bending and lead work. I use EXL 110 10-46 they come as a 3 pack for $9.00 on my Fender type electrics. I change them at least ever 2 gigs or every 6 months if I'm not playing live much. I put a sticker on the back of the headstock telling me the date and what set they were. 
 
I have a Guild arch top electric that I use Flat Wound Chromes on and they have lasted 2 years now ! They are ECG 24- 11-50. It gives that guitar a nice warm vintage sound. 
My acoustics I use 12-53 EJ 16 phosphor Bronze They sell a 3 pack of these $12. 
My Taylor came with Elixir and I just bought a new set, but after 4 months the originals seem fine..I'll wait and see. 
 
 

I lower the tension evenly on the strings before removing them, but pull them all off with each change. The reason is not for neck twist, but that the tension transfers to the remaining string(s). Leaving the high E for last can actually cause the string to snap, and that free end flying around can be dangerous.

Typically 4" off the fretboard (12th fret) is a good length to start winding. They DO change pitch initially, which is not due to the strings stretching, but the string taking a firmer wind on the peg. I typically tune, then bend each string as much as possible at the 12th fret, then tune again. Usually 3 cycles of this gets a firm wind to the peg (the eyelets will also seat more nicely into the bridge).

Watch the eyelets when initially tightening, as they should contour to the bridge. On the 335 they can cock and become a bear to pull out later.

Good point I actually do the same with de tuning all strings first but my post was long winded enough :) I also do the string pull which is important, especially Nylon which do stretch.  

If using 11's on electric you will want to decide first whether you want a wound or unwound 3rd (G) string, as they come both ways. Some brands of strings even include one of each type in a package of 11's.
 
I wouldn't worry about changing one at a time or all at once, but I would advise that you make sure you understand how to properly string them to the tuning posts.