Are you really willing to bet all that time to try and change a valve in a 15 year old compressor vs. putting in a brand new one that will likely outlast the car and work perfectly?

I understand there are cost considerations but I can't see any benefit to even trying this.

Regarding converting to a conventional compressor - my understanding is that making that change requires other changes to the system (I can't seem to recall which ones) but it wasn't as straightforward as just changing it out like-for-like.

I just kept it simple - my original compressor went 16 years and 160K miles before it started acting up. It was no more $$ to put the variable rate one back in, and it will likely never need to be changed again and it worked great for a long time, so the effort to swap didn't really seem to be worth it.