I've done it many times.
I just take the map, and flesh out the details one area at a time. Often I will change things to suit my purposes. The treasure especially I am likely to change to 3.5 standards.
One module for 1st level characters had assassins everywhere. In 1st Edition, Assassins were a base class like the Thief. Since it would obviously be inappropriate to make the 3rd edition Assassins, I made them Rangers.
One adventure was really terribly written. I threw out half the monsters, moved the evil druid to the back of the dungeon as the BBEG, and made all the monsters work for him. I also replaced all the teleportation portals with secret doors, and added new monsters to fill out the roster.
Most of my changes have to do with updating to modern standards. Gone are the "damsels in distress" who just look pretty while waiting for the heroes to rescue them. Everybody has stats and everybody has a motive. A couple of the "generic females" in the lair might be a priestess who can harass the PCs. One DM I had liked to salt the monsters ranks with rogues. The human druid or magic user might not need to be human.
So far I've converted "Against the Cult of the Reptile Gods", "The Sinister Secret of Saltmarsh", "the Pool of the Standing Stones", and a few levels of "Castle Greyhawk". I've been looking at converting "the Halls of Tizun Thane", but I've been wondering what to do about the harem girl.