Nice; how difficult was it to adapt? As I understand ot, that module is not super crunchy or combat intensive.
Critters - list is those referenced in I6; indicator is if in 5E MM
"Guardian of Sorrow"
"Maiden vampire" - standard vampire
"Strahd Zombies" - No
Bats: yep
Gargoyles: Yep
Ghost: Yep
Ghouls: Yep
Gypsies: use thugs
Horses: Yep - use draft
Huge Spiders: As Giant Spiders
Iron Golem: Yep
Nightmare: Yep
Red Dragons: Yep. Use adult.
Shadow Demon: Yep
Skeletons: Yep
Spectre: Yep
Trapper: No
Vampire: yep
Villagers: use commoners
Werewolf: Yep
Wights: Yep
Witches: no.
Worg: Yep
Wraiths: Yep
Zombies: yep
So that leaves specific NPC's, the witches, Strahd, Strahd Zombies, the trapper, and the Guardian.
Pretty straightforward.
The witches can be converted as Acolyte, swapping out the spells.
Strahd Zombies - convert the AC, note the flavor text, and note that multiattack number of attacks is number of times it's been hit. CR goes up by 2, simply due to multiattacks
Trapper: AC 3; MV 3"; HD 12; hp 62; #AT 4+; Dmg 4 + victim's AC; AL N.
Close enough to a mimic with AC17 instead (which ramps it up to CR3)
The guardian of sorrow - AC12, 10 HP, multiattack 7 - 6x boulder at +4 doing 1d10 each; 1x shake, Dex Save is DC 13 or fall. CR 3. Atts irrelevant.
The other NPC's: Use HP as listed, subtract AD&D AC from 20 to get 5E AC, pick a suitable weapon for listed damage, use commoner, bandit, or thug stats otherwise.
Strahd - Vampire +2 HD, +9HP, add spells as a 10th level wizard, Dex 22, AC18.
Also can call Strahd Zombies, and can call his swarms during the day, too. Probably Challenge 15. He is a badass.
Not the easiest conversion, but not hard.