Has anyone had success dusting off a very old module that you own like AD&D 1 Ravenloft, and running it using new rules and replaced monsters?
If so, any tips or things to keep in mind?
I don't know if it's a "success", but I have been doing that in a PbP game for the last year or so. I can't say which module it is, should some of the players have it and peek into it in advance

Anyway, I generally change names and vary locations and encounters a bit, so it may not be easily recognizable without a hint.
You don't need any conversion guidelines for monsters:
- use the 5e version of a monster if it exists
- if it doesn't, replace the monster with something similar that exists in 5e
- if you don't want to really replace the monster, use the
mechanics of a similar 5e monster, and still use the general description of the old monster
- if it's not enough because some key special ability of the original monster is missing, browse 5e spells until you find one that can work for representing that feature and change the fluff if needed (if the special ability is passive, you can often use it as-is)
So for example, don't chastise yourself if you can't convert Gwarfles from "The castle on the hill" module because you have no Gwarfles in 5e, perhaps Goblins or Kobolds are practically the same. If they are not because of a key feature of Gwarfles, there's a very good change you can just port that feature on top of 5e Goblins or Kobolds with minimal adjustments. But keep calling them Gwarfles and describe them as such!
For the exploration phase:
- use the guidelines about ability checks to make up reasonable DCs for exploration tasks and challenges
- if you encounter something that defies the rules system (e.g. a challenge that cannot be solved with a check, but only by doing something specific), keep it as it is... don't try to reduce it to a mere skill check, unless you actually want to lose the original feeling of that module/edition
Maybe you find a magic mirror that allows teleportation if you dance under the moonlight in front of it with a garland of onions around your waste. Do you really need a conversion for that?
