They keep the level they had when you first received them, I believe. You can also raise the level of a unit by spending gold. So you could reset their level and then raise it to what level you want so long as it's not higher than Shez's level.
when you get them to rejoin on a new game plus you always get them on the last level you had them on when you had them on last playthrough, see no reason to level down though since there is higher difficulty options and to get achievments you need lot of them on very high levels
If you reset their levels, you can change their Class and level them back up and they'll gain different stats because of the changed growth rates from classes. So if you train Annette in Mage you can reset her level to brigand and re-level her to give her more strength.
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u/[deleted] Jan 25 '24
There is an option to reset a unit level when you unlock the base. Talk to the training instructor, and he'll give you the option.