Person leaving: What sort of help do you need with this? The player is gone, so are you asking for help for what happens to the PC? If so, just find an NPC or something that the group has recently interacted with and state that the bard wants to stay and assist.
For the druid who wants to be a rogue: Just... allow it. Just let him rebuild and don't worry about it. You don't need to come up with a reason for him no longer using one set of abilities and start using another. If you still feel that you need a reason, then say the PC has abandoned his druidic ways and can no longer access those abilities. Instead, he now relies on his wits and reflexes.
In both cases, simpler explanations are always better.