What if Bite becomes a legacy move for Tyranitar?

Nobody knows for sure if any particular move will become legacy with this upcoming big change. The basic logic is you can definitely evolve the Larvitar and keep Bite as its fast move. If some new move is given to Ttar and is better than Bite, then you can always TM it to the new move. Nothing to lose.

Using rare candy or not is a different question though. Unless you really need THIS ttar as an attacker (I know it's your first Ttar), I don't think you'll need to spend your rare candy on it right now.

evolve but hold off on using rare candy

My standing speculation is that IF Tyranitar loses Bite, then it will be replaced with Snarl. This is directly comparable to Dragonite losing Dragon Breath and gaining Dragon Tail. Loses a short move that is good for dodging, but gains a longer move that is stronger.

The only reason to wait until you have enough candies to power it up to level 30 is the possibility that you might get a better one by that time. But that is not applicable if you are considering using rare candy now to power it up. The one you have is pretty good, so you can go ahead and evolve it now, and power it up further when you get the Larvitar candies. If T-tar gets new, better moves, you can always TM it, and if it gets worse moves, you will at least have gotten bite while it was available.

If you had, say, 150 rare candies, and you really had a need for the T-tar right now, maybe it would make sense to use rare candies, but with only 65, I'd highly recommend saving them for legendaries. Just evolve it now, and power it up as you get the candies. Maybe we will even get a double candy event for the holidays.

I'd evolve it now to get that done with, but don't power it up until you know the landscape of what happens after the first round of Gen3 (that will cost 125 Ttar candy). Bite could be replaced by a rock move, in which case he's not better or worse, just different. Best strategy is to have multiple Tyranitars, so go raiding if you can ASAP.

If raids are accessible to you, use of rare candies is quite replenishable. Therefore, I'd use rare candies on tyranitar if I need such an attacker now. That's exactly what I did for my tyranitar with 93iv.

Good luck if you choose to evolve now.