A question about how to convert a campaign

[Desril]

My GM has recently started playing AD&D, and wants to convert our current campaign into AD&D from 3.5. I'm inexperienced with AD&D, but I have yet to run into a TTRPG I didn't enjoy so I'm ok with this change. The problem lies with the fact that my character is a multiclased Crusader/Hexblade and, more importantly, a vampire.

I figure I can convert class wise just by being a multiclassed mage/fighter (by the way, how exactly does multiclassing work? Is it essentially the same as a gestalt in 3.5 but you only gain 1/2 xp? The book that I read never really explained it, just that only demi-humans could multiclass), but the problem lies with the Vampire part. Is a vampire PC reasonably playable in AD&D? Would I be horrifically overpowered if it is usable? How would we balance it?

[Diskhotep]

While I know of some people who have successfully converted campaigns, in my experience it rarely works out unless you are willing to accept some adjustments to your character. Too many times people focus on the minute details of "well my character has this minor ability, so it absolutely has to be in the conversion or it just won't be the same".

My recommendation would be to find the closest equivalent in race/class and pretend that the character was that way all along, using roleplaying to keep the flavor of the character rather than a checklist of everything he used to have (especially in the case of items).

In AD&D a multiclassed character is just that - a character who progresses in two (or three) classes at the same time. XP is divided equally between the two classes, making it possible to have a level disparity between the classes since XP progression varies according to class. Human characters must "dual-class", essentially quitting one class and joining another, forsaking their previous abilities until their new class level reaches the old one.

Since AD&D has no such thing as CL/ECL or Level Adjustments, any balance to your character from being a vampire is purely between you and your DM. Personally I would drop that portion of the character unless all the other player characters have similar advantages.

As far as an equivalent to your original character goes, the easiest would be an elf/half-elf fighter/mage or fighter/mage/thief. Even closer would be a fighter/illusionist if you are playing a gnome. A half-elf ranger/mage might also work. It's going to be all about the story, not the numbers and abilities after all.