Before I started the project, it was set to 48khz in the options.   If reson is in rewire mode the samplerate should be set by sonar, shouldn't it?

I know this is an old one...and an unusual question. Not necessary for most, but I have an old project that I'm reviving, and I recorded it at a higher resolution than necessary.
 
Is the best solution still to export each track at the desired resolution?
 
Thanks

Afraid so. Then create a new project at the desired Sampe Rate and import the audio files.

Thanks Mudgel

There is another option.
As Sonar doesn't destroy any data the audio files in your Project Name/Audio folder will be the unedited files that Sonar is using in the project in question, with the edit information contained in the .cwp project file. This is the case until you bounce/render the audio at which time a new audio file is created.

It is possible to create a new project, making sure to change your sample rate as desired then go to Windows explorer, locate the audio folder and drag all the files to the new project. That's it, Sonar should convert the files on import. You can of course just use the import audio file menu use ctrl +A selecting all files and import them in one go. Should also work.