In AD&D and earlier, PCs are balanced relative to each other by XP, not level. It was not uncommon to have the party thief be a level or two ahead of the fighter, who was themself sometimes a level or so ahead of the wizard. After 1st, a party of equal levels was not balanced!
That said, balance was not as important. These TSR editions prioritised in-play behaviour optimisation instead of before-play build optimisation, simply because your class levels gave you much less advantage than in WotC editions, and was often overridden by clever play. (The emergent result is that level was more often an after-the-fact indicator of player survival skills, rather that the cause of that survival.)
The upshot is that you can mostly ignore in-party balance concerns up through AD&D 2e. Original D&D through AD&D 2nd edition are largely intercompatible, especially moving forward through them. You should have the PCs keep their XP between those editions, and become whatever level that amount of XP makes them in the next edition.
For 3e, everything changed. PCs were nominally balanced by level now, and XP progression was unified for the classes. If you're going to average their levels, moving from AD&D 2e to D&D 3e is the time to make that adjustment.