When "updating" an m.room.member event, do we keep other fields?

The spec goes on and on about how things like `/kick` and `/ban` update the `membership` on an `m.room.member` event. Should the homeserver be required to keep all other keys when it does that?

A simple case is profiles, although some clients/apps may wish to add their own special keys to the event as well. The use case to force the homeserver to keep anything it isn't updating would be to ensure that there is always a profile for that user.

Start by reading the Matrix specification's discussion of m.room.member events and the membership changes used by /kick and /ban. Compare how profile and custom keys are treated, then clarify in the specification whether keys not being updated must be preserved; done means the rule and its scope are explicit.

Written by the indexing model from the issue text.