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

Some synths can't handle certain MIDI instructions. Some programs restrict the range of notes. More information might be helpful. Information such as the synth which plays properly, the synth which does not, the source of the MIDI data (IOW was it recorded in SONAR or a loop from a 3rd party), would be a good start.

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.

Another possibility is that you've swapped the synth over to one which needs a patch loaded in order to play, like Dim Pro for example

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.  

Perhaps its 1 or 2 or all the above. Seriously thought! You cant just change synths on an already created synth track, as each synth is different and may use only single out, different midi channels and so on.
 
If you want to change synths, make, tell sonar to insert a new synth on a new  midi and audio track or instrument track.
 
CJ

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.

I vaguely remember this happening to me the first time I tried True Pianos. IIRC it was some preference in the Advanced Interface page, like dynamics being set to 100 instead of 0 in the Options pop-up, or one of the Velocity parameters being misset.
 

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. 

okly dokly I will try that thanks. I do prefer the simple inst system as it allows insert effects without having to boucne tracks first, but yeah, I will give it a go :)
 
@Craig thx m8 - hadn't thought of that I will investigate...