I have played some of the great DnD games that were converted from DnD Version 1 or 2 into 3.5, but after you play through the Temple of Elemental Evil or Castle Greyhawk or even the Tomb of Horrors can you really reuse these?
The short answer is yes, but not with the same story, traps or monsters, most of the time. Take the city of Greyhawk for example. You can use this for any major metropolitan city in your world. Change the names of the quarters if you want.
The same goes for dungeons. Frankly, I find creating a dungeon to be the most time-consuming thing I have to do. So much in fact that I will create them off the cuff if I can with just a list of traps and monsters and have a handful of rooms for my mini-boss or my Big Bad. The other way to do this is to grab dungeons online and then replace the story, monsters, and traps with my own designs. Remove the flavor text and plugin what I need.