Overall I mean to maintain the feel of the module. Hypothetically, if after playing my conversion some of my players went back and played the original, I want them to think, "Yeah, I did just play through this." Doing this comes in several parts, which I lay out in no particular order.
1. Same plot, same locations. This is really a given; if I didn't want to play the same adventure or use the same maps, why would I bother with the module at all? Still, it deserves to be said that I don't plan on changing the background to fit the module into some existing framework. Rather, I'm running it in Greyhawk, though I don't expect the greater world to matter, since I'm making no attempt at continuity.
2. Same play-style. This was written as an open dungeon-crawl with set circumstances but no set plot. The conversion should play the same way.
3. Same difficulty. As I read it the module is meant to be hard and deadly, with the added twist of Gygaxian perversity thrown on top. This is no cake-walk, and PCs who are unprepared or overconfident die. I consider this a feature, and I've made it clear to my players that they probably want to bring two or three characters to any given session except maybe the first, so as to have backups.
4. Same obstacles. I won't be adding or removing traps of secrets, and I'm endeavoring to make the monsters fairly similar to the original. This ties back into point 1 pretty strongly; for me, a large part of the inherent feel of a dungeon is not only its history and architecture but also its denizens and operations.
Basically, the above can be summed up in the statement, "I want to run the original module, but since I don't know the AD&D rules and doubt I could convince people to play by them, I'm doing the next best thing."
However, there are some things I will readily change. First, number of opponents may get quite a bit of adjustment, due both to the fact that combat plays differently in GURPS and that I will have somewhere between seven and nine players, whereas the module was originally designed for six. Tactics of the opposition may also change, though they're pretty good throughout most of the module. Also, there are a couple spots where the deadliness morphs out into "rocks fall, everybody dies" territory that's just mean rather than fun. (Seriously. There's a passage that reads, "If the characters follow the tunnels off the mapped portion, allow them to proceed another hundred feet, and then tell them they are hopelessly lost." Full stop. They starve to death, end of story. And its baited to boot.)
Finally, wealth and reward is also going to change. GURPS and AD&D have completely different economics, meaning that conversion is possible but probably not desirable. I'll be taking the module as a starting point, but the party certainly won't be finding four gems worth $300 each under a farmer's rain barrel.
Addendum: What with Peter's kind post over at Dungeon Fantastic I feel I should point out that one of the things I don't plan on keeping the same is how Hommlet fits into Greyhawk at large, or, more accurately, I don't much care about Greyhawk. I won't be changing anything on that front (Hommlet and Nulb are still rather close to the Nyr Dyv and Verbobonc for example), but I also don't feel compelled to make my (minimal) representation of the outer world align all that well with canon Greyhawk.