I noticed something about the show that I thought I'd throw out there. The first few times they wrote off characters, it seems like the writers forgot to make episodes describing how they left by the time they were already gone. Think about it:
1. They waited like 6 episodes of Season 4 until they finally wrote off Midge, who wasn't in the episode.
2. Jo Ann wasn't in many episodes before they finally announced she had broken up with Bob, who wasn't in the episode, either, just "on the phone".
3. When Leo left, we didn't figure out what had happened until the middle of Season 5, and had to wait nearly 20 episodes I believe it was. Again, he just "left", and wasn't in the episode.
4. When Laurie was written off because she went to beauty school, she still had a relationship with Kelso, so the writers had to end it somehow. Remember on the Valentine's Day episode, Kelso goes up to her a room, a guy comes out, the relationship ends, and Laurie was never in the episode!
Obviously they wrote both Eric and Kelso out in the same episodes they left, but just saying about something weird about the first few times the show has handled writeoffs.