For the record, while this whole discussion of how much of a thing you can replace before it stops being that thing is all very philosophical, it's entirely beside the point of where this conversation came from.
I said Discovery wasn't a 23rd century ship anymore as a response to Angry Fanboy's claim that the spore drive was too ridiculously powerful not because of what it could do but because it existed too early (Relativity's even more powerful system is apparently considered ok just because it was from the 29th century).
The point is not that Discovery wasn't built in the 23rd century, it's that the spore drive was a one-off, super-powerful breakthrough that ultimately proved completely unrepeatable in the 23rd century and was never heard from again by anyone who stayed behind in that time. It's only in the 32nd century that it's actually being fully developed to a reproduceable technology.
Superpowerful unique techonological achievements that no-one seems able to reproduce are a staple of Trek, so the idea that the Spore Drive is bad because 'it's 23rd century' is just dumb, imo. Now- the fact that it could use a few more limitations, just for good storytellings' sake, that I can agree with, although the descriptions of its abilities in this thread have not been entirely accurate, either. Ie, accuracy is in fact at least somewhat an issue because their original time travel trip was not intended to be a time travel trip at all, and the whole 'you can jump 130 times with no cooldown' claim completely ignores that that little plan nearly killed Stamets, which would've rendered the drive forever useless afterward had it happened.