You have to decide that for yourself. Spray one of them and see how it goes and if it loses too much detail for your liking, strip them all.
Make sure you spray several thin coats, don't try and get them white in one pass, it might take 5 or 6 quick passes waiting a few seconds in between for them to tack so you don't flood the details.
I bought a bunch of 2nd hand Tyranids that were primed black and I wanted them white, I just sprayed straight over the top of the black and
IMO worked fine. I doubt anyone but me could tell the difference between ones I resprayed and ones I didn't, and that's only because I remember which ones were which not so much because the detail looks any different
But they are only painted to my table top quality and the black primer they came in was done relatively thin, if I were painting them for a display shelf I'd probably take the time to strip them (actually I wouldn't even bother buying them 2nd hand in the first place if I were intended to paint them for my display shelf).