freezing does not modify the source and is reversible. Freezing is reversible even after the project is closed and re-opened. Bouncing is similar to freezing ...the difference being is the original tracks remain untouched and a new track is created with everything applied to the wave track that results form the bounce....
HOWEVER.. if you have an audio track and have a plug or two in the FX bin and use the APPLY AUDIO FX option, the original track is overwritten with the FX applied and the original wave is gone. At the same time, to prevent doubling the effect, all the plugs and FX are removed from the bin since they have been printed to the wave.
You can still back up and use the EDIT / UNDO function to revert back to before the apply BUT.... only until you save the project and close Sonar. When you close Sonar, the undo file is dumped and now.... you can not go back. It is permanent.
Perhaps the original wave is still in the wave folder but you would now have to go back into it and search it manually to find the file or files which made up the original track and then manually drag them to a new track and place them in that track in perfect alignment to where they were to start with.... I say perhaps because I do not know for a fact that the original un-effected files are saved there and not over written.
For all intents and purposes, the APPLY FX function is permanent and done destructively to the original wave.