Another option (which I haven't tried but should work) is to export the clips as BWF files, erase the existing clips, then bring in the exported clips. Check Preferences to make sure SONAR recognizes BWF files.

OMG.. Mr Anderton..
Thank you Craig for YEARS of help!
You've been teaching me stuff since the 80's.
However, I think I'll give that last one a miss!! 
 
BR 

LongJohnBaldy
However, I think I'll give that last one a miss!! 
 

You might want to give it a try anyway...because there's an export option to export all selected clips at once, and you can bring them in to the project easily, it might save time compared to rendering clips individually.

Thanks Craig. But once I'm done with the fade in and out and happy with the checked clip and it's there in front of me, it's easy now to just render it right away with a (freshly assigned) bounce command.
Your kind response has, however been duly noted, photocopied, stamped and filed for future reference.