Can a partially collapsed hydraulic valve lifter be renewed, i.e., cleaned and replaced or must they be replaced new?

I wouldn't even try to repair a valve lifter - they are cheap enough to buy new, it's not worth risking causing damage, your local GM dealer may even sell them singly.
Malc

Reconditioning of valve lifters is a well-established procedure in the shop manuals. Dirt is the main enemy which can cause problems. Other main points: replace lifters back in the hole they came from and don't mix parts from one lifter to another. If you want to spend the time to do it, the shop manual prodcure pages can be sent and scanned.

I've only ever had anything to do on american engines for racing. I would never try to repair a lifter or any other highly stressed component for that. for street use Todd is spot on! go ahead & rebuild
Malc

Thanks fellas! Once pulled, I'll take a look at them and see what their condition is. If they're dirty, I'll clean them. If they have mechanically failed for some reason, I'll replace.