Minor, but prefer to check these strictly against null, e.g.:

if ($this->ids !== null) { ... }

This prevents a bug where callers do:

->withIDs(array())

...by mistake and accidentally select every result.

This might be worth breaking out separately at some point since I could imagine a user caring about the event getting rescheduled but not caring about the invitee list changing. Perfectly fine for now, though.