I've converted two older AD&D modules - Dungeons of the Slavelords and White Plume Mountain - into 4e for two Convention/Gameday Games. I've found three things:
1) The basic module converted VERY easily, because of the number of combatants in your average fight, and because of monster roles. Rooms full of 1 HD critters, along with a few 3 or 5 HD critters? No problem. There was even room to spice it up a bit by adding a trick or power to each creature that captures some of the feel the original designer was going for in a given encounter.
2) You HAVE to cut down those old modules for time constraints in a single four or five hour game. Even the original modules as-is in AD&D were not written for some four-hour session to complete - to get as far as you could to count points with, yes, but actually complete? No. Even then, I'm only getting half as far as I think I'm going to get, meaning I need to streamline even more. If you're not doing a convention game, though, spend all the time you want.
3)Some fights, and some traps from the old games scale VERY well (some in the A1 module come to mind here, as well as the metal plates in S2) and some are just plain bad or silly to me.
(A frictionless room and spikes with "Super-Tetanus?" C'mon!)
But 4E's exception-based design on monsters and traps jibes very well with those old AD&D adventures. If you want to reduce it to a skill challenge, fine, but otherwise leave it with those % chance to be screwed traps, if you wish. Me, I do wind up softening the traps a bit (Like a trap that if you roll 10% on, regardless of the PC, you're dumped into lava, that sort of thing) but only insofar as the "outright death through no fault of your own" traps go, because usually 4E gives you at least two or three rolls before it outright kills you.