https://www.gamefaqs.com/boards/370600-diablo-ii-lord-of-destruction/40829960
This thread suggests that since a certain patch, mercs no longer get a boost when leveling up manually, versus rehiring; however, hiring from "normal" does give a boost.
This thread suggests that since a certain patch, mercs no longer get a boost when leveling up manually, versus rehiring; however, hiring from "normal" does give a boost.