Perhaps just proving the YMMV adage, I just did this a month ago. Cancelled my wife's companion ticket and was able to reinstate the companion certificate by calling in a week or so later.

I am Diamond but didn't have the impression they did this for me because of that (nor do I get the sense that most things get done for Diamonds anymore). Mechanically speaking, the rep did have to separate the PNR's so that my wife's ticket could be cancelled without affecting mine. And she got a $30 refund representing the taxes paid on the companion portion.

I did not, however, use a RUC so can't speak to whether that makes it more complicated.