NPC conversion BECMI to 5e
by HåvardI think RobJN's suggestion of following the RC conversion to AD&D works well.
Personally, I am inclined to do a 1:1 conversion though, meaning 36th Level BECMI characters would be 36th Level in 5th Edition.
My main reason for doing so is not really about realistic power conversion, but rather that it would make the very high level Mystara characters such as Etienne d'Ambreville truly unique.
One would have to extend the Profiency Bonus table, which interestinly stacks up very nicely for 4 new tiers:
| Experience Points | Level | Proficiency Bonus |
| 21 | +7 | |
| 22 | +7 | |
| 23 | +7 | |
| 24 | +7 | |
| 25 | +8 | |
| 26 | +8 | |
| 27 | +8 | |
| 28 | +8 | |
| 29 | +9 | |
| 30 | +9 | |
| 31 | +9 | |
| 32 | +9 | |
| 33 | +10 | |
| 34 | +10 | |
| 35 | +10 | |
| 36 | +10 |
I am not sure about Hit Dice. By 5E logic it does seem like it would result in massive amounts of HP, even at 20th level.