If you’ve played through the other Fire Emblem games for the 3DS, you probably know to max your units’ current class levels before promoting them to the upgraded class to give them the best stats possible. If you’re doing this in Shadow of Valentia, you should stop because you’re doing it wrong. In SoV, instead of receiving bonus stats when you promote a unit, that unit’s stats are raised to meet the base stats of the promoted class. With the exception of a few more hit points, promoting at 20 will not have an advantage over promoting as soon as possible. Any abilities and skills you might have missed from your base class will be learned in your promoted class as well, so there’s no reason not to promote as early as possible.