Really? That’s interesting; I never heard that before. I always assumed taking a PC from one game to the next was just handwaved for the purposes of getting down to the dungeon crawl. Did people actually roleplay as if their characters were from separate greyhawk timelines?
Weirdly, the best written example comes from the FR Gray Box (DM Source Book)-
Bringing characters from other worlds-
...These characters may come over because of the demise of their native world, the discovery of a portal to the Realms, or the fact they have gotten too tough for their homeland. The last case occurs in Krynn, home of the Dragonlances, where individuals of incredible strength and power are politely asked to leave by the powers-
that-be. Many end up in the Forgotten Realms.
The world of the Forgotten Realms, Abeir-Toril, has a huge number of portals and other gate-type devices which link that world with other areas of the Realms, with the Outer Planes, and with alternative material planes containing other AD&D® campaigns. (Rules for adapting characters follow).
The rules for AD&D are contained within the DMG,the "Campaign Section" (shortly before SIxguns & Sorcery and Mutants & Magic section) pgs. 110-11, discussing integration of players from other campaigns.
See also-
Thus, in my ”Greyhawk Campaign” I included an “Alice In Wonderland” level, and while it is a deadly place, those who have adventured through it have uniformly pro- claimed it as great fun because it is the antithesis of the campaign as a whole. Similarly, there are places where adventurers can journey to a land of pure Greek mythology, into the future where the island of King Kong awaits their pleasure, or through the multiverse to different planets, including Jack Vance’s “Planet of Adventure”, where they hunt sequins in the Carabas while Dirdir and Dirdirmen hunt them.
But as a matter of course, this is usually hand-waived unless there is a major difference in settings.