Can a rockshox pike be lowered to 120mm then reverted back to 140mm ?

Yes, you need a 120mm air shaft (suitable for your flavour of fork), keep the old one to convert it back afterwards. Lots of video guides around showing you how it’s done, have a search. Got an unused 120 29er air shaft hanging around somewhere if that of any use

Mine can, but I have old skool U-Turns.

Sorry no help if you’re talking about new ones!!

Can you still put spacers in? I’ve done that with a few sets of rockshox before.

Zippy has it, you need an airshaft the right length for the fork travel.