Unfortunately the SONAR-only procedure has some big shortcomings: It doesn't address bringing the audio back into the
original project with the new sample rate, preserving all the non-destructive processing and other project content and settings. And SONAR renames the files on export in a way that can make that difficult to re-import everything manually. Plus it forces you to bounce down multi-clip and multi-lane tracks to single-clip tracks.
This is where a 3rd-party sample-conversion application like the free Voxengo R8Brain comes in handy. R8Brain has a batch mode that will create re-sampled copies of all the audio files in place with the original names and add a prefix/suffix to the original files (or delete them if you so choose). Then when you re-open the original project (or a copy of it) with those re-sampled files in the audio folder, SONAR will automatically set the project rate to match the audio it finds, and you're done.
Though it does involve using a 3rd-party conversion app, it's by far the fastest and least complicated way of converting a project's sample rate and/or bit depth without altering the project otherwise.