Room upgrade to room with different settings

**Is your suggestion related to a problem? Please describe.**
I have a room which is encrypted, and contains several users who are less comfortable with technology. I need to upgrade the room, and would like to be able to send an upgrade notice. However, the room population has grown, and I would prefer for my new room to no longer use e2e.

**Describe the solution you'd like**
When I perform `/upgraderoom x`, an additional, optional flag to make different settings in the new room, specifically encryption.

**Describe alternatives you've considered**
I could craft my own tombstone event and use that, per https://matrix.org/docs/spec/client_server/latest#m-room-tombstone.

Start with the /upgraderoom command and the Matrix m.room.tombstone specification linked in the issue. Determine how an optional encryption setting should apply to the new room and upgrade notice; done means users can upgrade to a room with different encryption settings without crafting a tombstone event manually.

Written by the indexing model from the issue text.