Ok, I did some SI strings.
Now I'd like to do a bass track
So how do I tell the keyboard to now use the SI bass instead of the strings?

The easiest way is to just insert another synth track, using SI Bass instead of the SI Strings.
(otherwise setup just like your original SI string track).

Click on the track (your SI Bass track) you want to play to make the "active" track, and that's the one that will be using your keyboard input.

Input echo.
 
It should change depending what track is in focus, otherwise we may have to tweak a few things.

Check out this free video for some info on working with synths in SONAR...

* Cakewalk SONAR: DreamStation Synth Demo
http://www.digifreq.com/digifreq/video.asp?ID=9

And some more SONAR videos here:
http://www.digifreq.com/digifreq/videos.asp

Scott

--
Scott R. Garrigus - http://garrigus.com
* Author of the Cakewalk Sonar and Sony Sound Forge Power book series: http://garrigus.com/?PowerBooks
* Author of the Cakewalk Sonar ProAudioTutor video tutorial series: http://garrigus.com/?ProAudioTutor
* Publisher of the DigiFreq free music technology newsletter: http://digifreq.com/?DigiFreq
* Publisher of the NewTechReview free consumer technology newsletter: http://newtechreview.com/?NewTechReview

Aha! Input Echo was it.
Also I needed to set the strings input to nothing.
And thanks for the video links Scott

I don't think I have ever touched the input echo button when using soft synths. Doesn't it just select itself when you select the track? If you're just using a single keyboard, make sure the MIDI input of each instrument track is set to Omni. 

I had opened a new track with SI Bass but the keyboard continued to play the old SI Strings until I set their Inputs to None and the SI Bass input to Omni.
The SI Strings were not echoed but played until input was set to none.
The SI Bass would not play until echoed.
Also the SI Strings reset themselves to omni every time I close and open the song

If you have all of your instrument tracks set to omni, the "active" synth should always be the track that is selected. You should never have to set any of the tracks to "None." Also, the input echo should turn on automatically when you select an instrument track, and turn off when it's deselected. Are you using Instrument tracks (combined MIDI and audio track), as opposed to setting up separate MIDI and audio tracks and linking them (the long way of doing it)? 

Would that just be X2? I'm still X1
When I open sonar, the first SI is enabled and plays thru the keyboard, all the SI tracks are set to omni and it does not matter what track is selected it still plays the first one.
It was installed with the one check mark to the simple instrument track.

Your description of the set up makes sense and it would be nice to figure out why it doesn't.

There is an option in Prefs-->MIDI-->Playback & Recording called "Always echo current MIDI track". 

Make sure that is checked if that's the behaviour you want.