Trust me I understand the confusion. And, just to create a little more confusion, I submit that both are true. You probably should restart a campaign with the release of 4e and it is easy to convert older adventures and it is possible to convert exiting campaigns.
Okay, before you beat your head in frustration, hear me out.
The game is different enough that it’s a very good idea to start a new campaign upon release. There’re enough differences it behooves any DM or player to start with the basics and work up from level 1. It’s also impossible to take an “X”-level character and rebuild it in 4e terms if your measure for a rebuild would be absolutely equivalent abilities at each level. You can convert rather satisfactorily if you measure is more loose; more about tone than detail.
For instance, my Rise of the Runelords Hellknight is a tank who challenges his foes and layers the smack when needed. He doesn’t have the same exact feats, the same class features, the same equipment, but he does have the rules items in 4e that allow him to do what he is famous (at least in my own gaming group) for doing.
In essence he’s the same character. His mechanics are not identical…just similar enough where I feel like it’s the same character. Keep in mind, he’s not the most complicated character in the world, but the other players have been able to convert their characters with similar results.
On the other side of the fence, I would hate to try to convert Living Greyhawk characters. That would just be an utter charlie foxtrot, as too many people who wouldn’t be happy with the conversion. That campaign is just too rich, deep, and played for a tonal conversion.
On the DM side, when building tools for the DM, Stephen Schubert (the ever-lovable, Shoe) created interesting ways to express game assumptions that grant a tremendous amount of flexibility on the DM’s part for creating and modifying challenges. Those revelations make it easier for DMs to tell stories while still supplying the gamist challenges that the majority of D&D players crave. I find it easier for a DM to create adventures in 4e and it is also easier to convert. As for particulars, I’m afraid you are just going to have to wait for the Dungeon Master’s Guide.
Now, here’s my big caveat to this. If you are a DM who uses or wishes to use the D&D rules for a physics of the world, where monsters and players always live by the same rules and building monsters and NPCs is a rigorous and time consuming as building PCs, the ease may make you feel a little queasy at first. The truth is, it’s just a heck of a lot easier to create monsters and NPCs, because you build them enough for the role they are designed to play in the plot and in the game. And we really latched on to this philosophy in monster, NPC, and adventure design. Like characters in a movie or play, they are fleshed out as much as they need to serve the story—in this case the adventure.
So basically, I think it’s best to restart so that you and your players can get to know the rules and how they work in a holistic way. This doesn’t mean you can’t run Pathfinder, Temple of Elemental Evil, Age of Worms, or Sunless Citadel as part of your first campaign, and supply the same story and challenges that those classics provide. Of course, H1 Keep on the Shadowfell is pretty fun too, so you may just want to start with that one. Anyhow, as a DM, your gaming world in 4e is pretty open.
Stephen Radney-MacFarland
Developer dude.
and raise you a 
