It would be great to have an option similar to the existing 'Change Audio Format' bit depth changer in the utilities menu but instead to change a projects sample rate. It could just use Sonar's built in sample rate conversion algorithms. And for those users who are picky about using a third party product to convert, you could always make a copy of the original audio files to convert them how you like and then replace them.
 
There is a manual workaround, but it only works properly (kind of) when downsampling your project - please see recent discussion in this thread:
 
Upsampling projects using this workaround end up with silence at the end of each clip of length depending on the ratio of old sample rate vs. new.
 
Even an option that allows users to ask Sonar to recheck all audio files tied to a project and find/update their length would be a good enough workaround as the results in the above mentioned thread indicate that audio file lengths are stored in the project file instead of being calculated on open, and dictate what Sonar can see when dealing with clips.