Replace Synth on a track and now no sound?

I would have thought this command is as simple as it sounds. No hoops to go through simply replace the synth and the part already recorded plays just the same except through the new synth...but apparently not!
 
Any suggestions?
 
Thanks

One possibility: Some synths have their own selection for MIDI channel. If the  default channel of the replacing synth doesn't match the one of the original track/routing settings, you need to change it separately.

Perhaps you have not figured out how midi tracks need to have their output assigned to a synth. 
 
You can insert as many synths as your system can handle into the synth rack. 
Some, as said, need to have a sound patch assigned before they will play. 
Then you can take any midi track and by just changing the output as it plays listen to what the different synth's sound like.  

thanks guys...in this case it was a simple inst track using UVI Ravenscroft, but I began having iLok (software version) issues and it became unavailable- so I thought too easy, I'll juts swap it out for True Pianos (my full version not the Cake one)...and nada! I even created a new TP insert and simply copied the midi data, and still nothing..so I am doing something wrong or missing something.

Copy the data to a regular midi track. That will rule out the weirdness of using a simple instruments track. 
Insert TTS-1 and point the track at that to test. 
Then try True Piano's. 

There is no requirement to bounce tracks to insert effects in the audio track of a MIDI+Audio pair but I doubt the problem is the track type