Are there any rules for DNS root zone file changes?

Solution 1:

This is documented in IANA's Root Zone Management, in particular the Change requests → Procedures and guides section.

It lists the Technical requirements that any change must conform to, and while the amount of changes isn't mentioned as a technical requirement, it does mention that changes which actually attempt to switch nameservers to a different operator would indeed be handled by a completely different "re-delegations" procedure:

If the request is deemed to represent a substantial change of control of the TLD, it is considered a redelegation request, and must be assessed according to the criteria of that process