I have 2 RXPs. I put a valve train upgrade on one of them and I want to move it from one RXP to the head on the other RXP. The upgrade has <10 hours. Is this OK to do?

I heard someone say that the retainers take a set so you shouldn't move them. Can Riva TS advise on this issue?

BTW - I do NOT want to swap heads. Just the valve train upgrade kit. Can I do it? Any reliability risk?

You can do it. But, you will need to purchase 6 more of the bolts that hold the rocker arm shaft in place. They're "stretch bolts" and should not be re-used.

I wouldn't swap the retainer keepers (valve cotters). Move only the retainers, springs and shims (valve stem seals).