I'm going to take a shot at answering this one. But the question is both broad and tricky. So you may see me conjure up a big wall of text.
Conversion? It'd be a quagmire...if you think of conversion as something essentially formulaic. The conversion document we did when 3.0 came out had a lot of "If your Strength was 18, now it's 20" and so on. That won't work this time around. And I'll contend that back in 2000, that conversion document only let you limp across the starting line. I'm pretty sure that most people would have been more excited and a lot less confused if we'd have just said in 2000: "Seriously, guys, finish up what you're doing and then start over."
I'm staring at the pretty much the same bookshelf full of 3.5 books as the rest of you. It's huge! There's just no way that a formulaic/algorithmic conversion process can cover all of that. And here's a question back at you: Even if gave you such a process, would you trust it, or would you do your own thing anyway?
So I'll offer up an alternative to conversion: reinterpretation. Between the core books and the stuff on Insider, you should have the raw materials to "faithfully reinterpret" your favorite 3.5 characters. Their stats and various mechanical elements? Many will be different. But the fundamentals--who your dude is and what he does in time of peril--will be the same.
Maybe an example will suffice. I've long used my Thursday night buddies as a testbed for whatever crazy D&D stuff I've been working on. And a few months ago, I dropped the "we're switching to 4e...tonight!" bomb on them. Half of these guys are my fellow designers, so they know the rules as well as anyone. But for the other half, this was out of the blue.
They got a 20-minute whiteboard lecture from me on the fundamental rules changes. Then they got all the character creation rules and the combat chapter. I told them to start reading, then "faithfully reinterpret" their 3.5 characters as best they could. 90 minutes later, we were playing.
Were they converted characters? No way. We had only eight classes and a handful of races. More than a few PCs have new races, new classes, or both. Likewise, they completely re-equipped. But if you squint, Bruce's character is doing pretty much the same mindblasting stuff he always did. Toby's character is still the fulcrum around which the party's tactics revolve. And Cam has another in a long string of brave but lightly armored characters with a penchant for mischief.
It's worth noting that on that night, I had some significant advantages. I had three other designers who could whip up their new characters quickly, then help the guys who were coming to terms with the new rules set. And I'm a little sad that my Thursday night guys didn't get a really kickass low-level experience, starting out instead at 7th level. But it worked, and our campaign rolls on.
So formulaic conversion? I can't see a way to make that work. But a faithful reinterpetation? I know that works because it worked at my table. You just have to be willing to squint a little.
--David Noonan, game designer, Wizards of the Coast.
