Intro: If you autolevel an enemy/Green Unit with a weapon that they do not have the base rank to wield, the game will give them the weapon rank to wield it.
IE if you had a soldier with base rank D and gave them a Spear, it would give them a B rank in Lances.
HOWEVER:
If you give an enemy a weapon with a lock on it (Such as a Rapier), it will not autolevel them. Normally this would not be an issue, due to normal Prfs in FE8 having no rank. However if your hack has a weapon with both a Rank and a Lock, it will not autolevel them.
This can be fixed by going to 0x17f88 and writing 0x00 and 0x21 there.