They were always going to vote for him and will always vote for him.
The election is going to get decided at the margins by the kind of people who can still get swayed one way or the other to vote or not vote for one or the other candidate.
Actually convicting him can give a solid reason for someone who would have only very reluctantly voted for him to switch to Biden or just not vote at all. Old school Reagan conservatives who find that voting for an actual convicted felon is something they just won't do. It does create an additional nice red line of illegality, which should still matter to a few folks. It will cause some ethical problems with some Republicans for voting for him, and it really doesn't take much. The 2016 election was decided by a few 10,000s of people in 3-4 different states.
Where it can backfire, though, is if the conviction is overturned on appeal.