I converted Castle Zagyg: Upper Works to 4e and tried to stay as true to the text as possible, only making a few blanket changes to the size of rooms & corridors. I found that the changes were surprisingly minimal: as long as the terrain was *interesting* then it worked for 4e just as it did for C&C, which means 3.5 can't be that far off either.This sounds like an awesome decision, until you realize 4E is a completely different game than AD&D/3E/Pathfinder/d20/oldschool/BRP/Dragon Age/RQ/whatever.
The way you design encounters (space, terrain, monsters) and the way an adventure needs to be spaced operates under completely different requirements.
You simply have to design to either 4E or other systems if you want to end up with a good result for the target system in question.
Creating a generic adventure and then supplying a 4E appendix simply and sadly will not work.
Not unless you know something I don't. And not WotC designers either.
Basically, I agree that 4e is very different, but I do not agree that it is completely different. You'd have to have your baseline adventure assumptions lie somewhere in-between the 4e mindset and the "any other edition of D&D" mindset, and then you could probably serve both pretty well. A 4e stat PDF tied to the adventure could get by on saying "Increase every room/corridor by 1.5, rounding up in order to get full squares" or something like that.
IMHO, the 4e mindset of "more interesting terrain and non-static encounters" is great, and I've been using it since long before 4e was a twinkle in anyone's eye. Looking back at some old adventures, others subscribed to that theory as well, as many of rooms are both interesting in terms of terrain, as well as non-static in terms of drawing encounters from other areas in the instance of combat, noise, or other actions by the PCs. 4e makes some of this more implicit in the rules, but it's generally just good design that can be applied across editions.