One of my players played Planescape: Torment, and recognizes the occasional location, NPC, or reference. Otherwise, the players’ knowledge is mostly governed by what I tell them, or whatever Planescape ideas survived the transition to 3rd Edition.

When I first conceived the campaign back in 2nd Edition days, I deliberately chose one of the most obscure MacGuffins I could find, so I wouldn’t be contradicted by “continuity” (I spend a lot of energy describing the Night Masks of Westgate in Forgotten Realms before TSR did, for example). But now that I’ve made the setting “mine,” I’m willing and able to make drastic, sweeping changes if they’re called for.

The DM in the last FR campaign I played in wasn’t afraid to make changes, either – the monarchy of Amn was restored, and massive crystals were swallowing the capital of Cormyr when the campaign stopped.

I love the Forgotten Realms; it’s my favorite fantasy setting by far. Over the years, I’ve had the good fortune to run it for folks who either knew less about it than I did, or who didn’t care that I treated it as my personal sandbox rather than as one of the most documented, canon-filled gaming worlds ever created.

Looking back, I’ve always made the point of saying that I wasn’t running the Realms quite as written (just as you suggest, Telas). That helped a lot.

I also agree that changing stuff in a published setting is a lot of fun, and it teaches you about the setting in a different way than just reading through the setting books does.

If the players are more in the know about the world you’re using, you can always mine them for plot ideas. For instance, the party finds out a dracolich is rising out of the desert to try to Take Over The World. The players decide to run to hide under Elminster’s robes, which you had not anticipated. Throw in a few encounters to tap out the rest of the session, then when you next meet, you can have the encounter with El tricked out and fully-prepped (or just be ready to tell the party NO! as a certain cranky old mage magically whisks them away out of his hair).

My main issue with using pre-established campaign material is that some players use crutches and stifle their own creativity. If you’re announcing a new Forgotten Realms campaign, odds may be pretty good someone will try to either make a dual-scimitar-wielding drow who ISN’T evil, or some other flavor-of-the-month character based on someone they read about. If you don’t have people like that, you’re blessed. But I have an issue with copying exactly someone else’s ideas for characters, and I find that as much as anything pushes me to creating my own material and worlds.