Say for example, I record a stereo guitar track into Sonar.  I then add a few stereo fx on the inserts.  Later, I decide to convert the guitar track to mono.........   To get everything working correctly in mono, do I need to switch the Stereo/Mono button to mono and replace the fx with the mono variant, or is this done automatically by Sonar?  I've noticed that the Stereo/Mono button does not change automatically after conversion.

You need to replace it all.
 

I'd say it depends.
 
If you route your guitar(s) to a bus and set IT'S interleave to mono, or insert a strictly mono plugin on the bus then everything will be heard in mono yes?

I'll get more into detail-  the stereo recording will merge into a mono one if you click the mono button, but you will have no control over how the waves sum so you may not like what you hear, I've had this happen.
Many plugins have a mono button and that is all you have to do with them, others will have to be replaced.
 
Usually the first thing I decide on for a track is weather it is going to be stereo or mono so I don't run into these types of problems. Sometimes I will record into mono and then decide I want it in a stereo track because of a particular effect that sounds good in stereo and not wanting it on a send. Mono is fine on a stereo track, but stereo recordings shouldn't be converted to mono normally. At least that is what I have found in my years with Sonar.

I guess, I'm going to need to take this into consideration as well.  I have my Kemper Profiling amp connected via SPDIF, which allows me to hear and record the onboard effects into Sonar in glorious stereo.   Many times though, I'm just laying down a simple guitar part with no fx, and I end up converting the guitar track to mono.   Guess I'll just record them in mono from the start, to avoid forgetting to change the stereo/mono button, and swapping to mono fx once in Sonar. 

When adding a new audio track in Sonar, does it default to Stereo?  If I change the input channel from my interface to mono before recording, will Sonar recognize this and adjust the default stereo track to mono, or do i still need to remember to do this manually?

davec69
When adding a new audio track in Sonar, does it default to Stereo?  If I change the input channel from my interface to mono before recording, will Sonar recognize this and adjust the default stereo track to mono, or do i still need to remember to do this manually?

sonar will record what you are throwing at it / audio track wise.
if you plug your guitar in to an interface with one input, click record in sonar, sonar records a mono track.
if you route 2 tracks, a left and right to one track in sonar, it will record in stereo.
 
but from mono to stereo FX is where it gets interesting.
if you apply a mono signal to a stereo effect, sonar will change the signal to stereo output.
your recorded file does not change, just the way your FX handles the signal does.