Hi guys,
I recorded a full song with the bass line "legato" (smooth, lots of sustain between notes) but realize now that it would fit the song much better and less muddy if I had it done "staccato"  (punchy).  Can't seem to make the alteration with what I have or know at this point. I would like a rather dramatic change. 
 
Any way to change the bass with a plug in or something, rather than a whole re-record ?

Don't laugh at this idea until you try it. I have now converted almost all my songs over to MIDI bass using the 
Drag and drop Melodyn trick. I can get the part precisely the way I want it using Midi editing. Bass is one of the trickiest instruments to dial in so it's consistent from song to song. Midi solves this issue for me on my backing tracks as well as many of my recordings where I use a lot of Acoustic bass. 
 
Just give it a try you have nothing to loose.
Insert a new midi track
Drag and drop the bass track to the midi track and melodyn will convert the audio to midi. 
There are hundreds of choices for bass VST's I use the SI Bass and some Dim pro Acoustics.
With the SI bastweek it a bit. It can sound pretty close with the right treatment. 

+1 to the above. One caveat with SI bass, I think that is off by an octave from the MIDI you will get from a drag and drop so you may need to transpose it if you use that.

You could use a gate side chained with the drums buss and set or automate the attack, release and threshold to fit.
 
Rocky
 
 

Gee...  that's a great idea with the midi conversion...  thanks !  Will take a lot of work, but may just do the trick. Only problem is the sonic difference and feel between the original bass lines, which will be kept as played by the bass player, and the midi lines. Not sure how a CD would sound (as far as getting a consistent sound when mastering) when just one or two tracks are midi and the rest are real. 
I did try a gate, but the result was terrible...  perhaps just my ignorance.

The thing about the midi conversion is the best of both worlds. Normally midi is entered from a keyboard controller or manually drawn. This will not come across the same as the way you would play it on a real bass.
 
I can lay down a bass track and convert it and edit into exactly what I wanted in way less time than it normally takes me to fuss with the bass tone, then the actual performance has to be perfect. With this method you play it as best you can and don't have to worry about tone, string buzz or small mistakes. So it takes what could have been an hour or more down to a few minutes. 
 
The editing if you are handy with the PVR is where you can polish the track to perfection and for me the end result is a better overall track than what I could have accomplished in that time period. 
The sound, tone etc of bass in a rock mix is a matter of personal choice and of course there's no substitute for the real deal. But if the real bass is not satisfactory you would have to re record it properly. To me there's no turd polish. A good recording starts at the top of the chain, not the bottom. 
 
VST instruments are samples of the real thing and there is no difference audio wise in a mix. It's all in how you manipulate them.
 

OK, I gave it a try...    I dragged the Region FX melodyne'd clip to a midi track (SI-Bass) but it doesn't play right or sound right...  the simple bass line (open E) - G# - B - C#  only plays the last 2 notes, B- C#, and even in the midi window I can't get the first 2 notes to sound at all...  ?    I tried to drag it to Rapture LE but it won't produce any audio at all...  something I just don't know about.  Any ideas?
 

You missed the comment above by Mettelus. You need to process/transpose +12 
 
The down side of this whole idea is if your not adept at midi editing you will have a whole new world to learn which might be daunting at first. Some of us it's second nature as we been at it since Cakewalk was only midi. So the results will depend on your midi editing chops.  

Some great ideas on this thread. I also Dupe Bass tracks with Synth bass as well.
The one thing that Melodyne fails to do well is velocity changes.
I do a lot of hand muting with bass and sometimes an occasion fast attacking notes.
Melodyne has no clue when it comes to those two things. Some hand work is necessary. IE- Fine tuning.
 
This is a good example of doubling a Fender P-Bass with a Midi Synth bass. Listen to the intro-
https://soundcloud.com/charlie-roy/seven
 
P-Bass Is direct/clean. Synth Bass has saturation/dirty.
Probably not your cup of tea but that's how I do it.

Cactus Music
The thing about the midi conversion is the best of both worlds. Normally midi is entered from a keyboard controller or manually drawn.

What a great idea to use Melodyne, Cactus! I was always entering the MIDI manually (okay I have my own program for that) when it took me too long to learn to play the bass line perfectly. By the way I like to use the Scarbee Rickenbacker bass VST!