Haley's player (lets call her Harriet) can't make it to the thursday evening d&d sessions for a while 'cos she's doing an evening class, or some other regular thing. Eventually she'll finish them, but for now it's decided she'll game on a wedsday instead. Bob (belkar's player) and maybe roy's player, Rob, (taking over Celia) decide to play on wednesdays for a while, but everyone else is as normal, and the dm runs 2 sessions a week. While this is going on, Leslie (V's player) drops out of the group, meaning the DM takes over as V.
(and to join in the mass prediction making, I reckon #600 will see us switch over to Haley.)