Well, 2Ed was fairly compatible with 1Ed, for instance.
And as for 3Ed, while it wasn't perfectly compatible,
- WotC was kind enough to publish a 2Ed to 3Ed conversion manual that made conversion very easy
- The degree of incompatibility from 2Ed to 3Ed was MUCH less than from 3.X to 4Ed.
By way of example, I'm part of a group that has run a campaign since 1986 or so. We converted...well, really,
alloyed 1Ed & 2Ed, and when 3Ed came out, we started converting to that. Now, I know I had problems with converting some multiclassed PCs, but none with single or dual classed ones. And even the multiclassed ones didn't really lose functionality or capabilities, they just lost a class because the remaining ones did the job nearly as well.
There simply wasn't a PC we couldn't model in 3Ed out of the box. The only question was fine details.
But converting from 3.X to 4Ed? Not a chance!