I think with early voting you can show up on election day, nullify your mail ballot, and vote. IINM that's how they answered the charge that early voting would create a disaster in just such a case as this one, with a potential felon being elected based on votes already cast prior to the crime.
Apparently not in Montana
Unfortunately, Montana does not allow those who voted early to reconsider and vote again. We're one of the few states that does not. This would seem to be the best reason we should urge our state leaders to change that law.