CHANGE NOTHING
Think about it 'in game'. Just because the Sorcerer decides on a career change, the world does not change the direction of its spin just for him. Losing a player can be traumatic to a gaming group. Why not let the characters also work though their loss in game, finding things to be tougher, having to play smarter.
We lost a gamer before starting a new campaign about 6 months ago. We just decided to press on, and wound up doing more roleplaying and 'playing smarter' than in any previous campaign with that group (and no, it was not like the departing player was the only H/S'er). When you have a group of 7-8 characters, no one character is terribly important. When you have three (which we did), even the loss of one character is devastating. It gives the players more of a sense of ownership.
The other option is that everybody plays two characters. Might be hard if you've never juggled before, but in a group where everybody is or has been a DM, it is not too tough. That way, each player can have two specialists (spellcaster, roleplayer, combat machine, other wierd ideas), and one is always in play. Out group with 3 players and 6 characters was the first time we ever used the Aid action for skills or even in combat. Just another way to make people think a bit.
Apologies if you found this post to be a rambling, directionless mess. That's just the kind of day I'm having.
-Fletch!