Transitioning campaign from homebrew to published setting

[Sir Whiskers]

Has anyone ever transitioned from a homebrew to a published setting before? ...(snip)... Lucky for me the game has so far been limited to a single city and three geographical regions. And I've got a dialog going with my players about the shift, to which they are amenable in the abstract.

I'm still brainstorming and sleep-deprived, and have only half-formed thoughts, so I'm wondering what ideas or experiences you might have on how best to do this?

I did something like this in college. Basically, the players had a keep in the wilderness. I stole an idea from an old Jules Verne (?) story and had a comet sweep down, dragging the keep to another world. You could do something similar, if you want to keep the current city and regions. A spell gone awry, causing planar shift could have the same effect. If you don't care about the current setting (the city and such), you could have the PC's simply travel into a series of caverns, only to emerge into a completely new world.

As for mega-module suggestions, I'd go with the Vault of Larin Karr by Necromancer Games. I've also read good things about the Grey Citadel, though I haven't run it. I'd only recommend RttToEE if you plan to massively tweak it - since you're trying to save time, I don't think it will fit your needs as well as other options. All IMHO, of course.