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.