rounser said:
And if you believe that, I have a bridge in the Free City of Greyhawk to sell you.
It's a non-trivial project to convert from older editions to 3E. Stats are indeed the crux of it all. Old edition monster not out in 4E? Hmm, where's a viable substitute....oh, and four goblins were a challenge in 3E, but in 4E we'll need 12 etc. By the time you've done all that sort of stuff, you begin to ask yourself why you didn't just write something from scratch. You might re-use the maps and the plot, but all those monsters, traps, treasures and puzzles will probably need time consuming translations, or workarounds if there are no direct equivalents as some cases.
Worldbuilding, again, is not a rule-dependent process. End of story. I notice you have stopped talking about worldbuilding here, so I assume you've given up that line of argument.
As for adventures, we have been promised that the DM's job of building monsters, planning adventures, and other bookkeeping tasks will be made significantly easier. One of the examples of this we've already seen is that it will be possible to build new monsters essentially on the fly, using the monster creation rules. If it's anywhere near as easy as they say it is, I don't suppose it'll be too much of a problem to look at an existing monster and update it. Certainly we won't have to do all the work 3E requires with HD, skill points, feats, etc.
Take the Mad Slasher, a monster that appears in The Whispering Cairn. It is a quick monster with moderate HP, a moderately damaging attack, and an attack that lets it hit each adjacent opponent once. Its role is something like "ambusher," or whatever they're planning to call it in 4E. So, I look up the stats for a 2nd level ambusher, slap on the all-around attack power, and I'm done. Do I need three more of them for a balanced encounter? I cross off "1" and write in "4". Did it have a wand of CLW for treasure? I look up something equivalent in the DMG. Max five minutes work all around, if the previews are anywhere near accurate. While true conversion is unlikely to be easy, replacement of monsters, treasure, and other non-fluff elements with their 4E dopplegangers is probably going to be.
There are two reasons I buy prepackaged adventures. First, I don't want to have to spend a lot of time working up 3E statblocks. That will probably be, to a greater or lesser extent, remedied in 4E. Second, I don't have the time to write internally consistent plots, realistic NPCs, or plan dungeons. These things don't really have anything to do with the edition I'm using. I could run Age of Worms in FUDGE if I wanted to, with basically 15 minutes of prep per adventure. It would play a bit differently, but all the elements that make Age of Worms interesting would remain intact, and untouched.