If you already own it on steam can you now get the meta version or is it only for new purchases?
as already said no sadly you cant.
there are 2 different things going on here.
if you buy a game on the meta PC store (formerly oculus store) or the meta quest store then some games are cross buy and you get a quest AND a PC copy of .. Beat saber didnt used to be cross buy but is now.
then there are the 2 (or more ) different "rival" store fronts which are never*** cross purchases... so if you buy on steam, or pimax, or vive store, then you wont get a meta copy.
*** pedantic perhaps as not that relevant now but back in the day sometimes the developers would manually send you out a code for a different store front when this split 1st started happening, but this was before VR was mainstream really... i highly doubt they would do it now........ Eleven table tennis devs are one example of this.. but again, afaik they are not any longer.