Linked JIRA provides the best overview of the whole issue and also contains a reproducer.
*** Bug 1104657 has been marked as a duplicate of this bug. ***
*** Bug 1104658 has been marked as a duplicate of this bug. ***
This need to be documented as a known issue.
Tomas Sykora <tsykora> updated the status of jira ISPN-4343 to Resolved
This is not a bug. Closing.
I was able to successfully proceed REST rolling upgrades from 6.2.1.GA to 6.3.0.CR3! Tests are using REST helper. There is NO compatibility mode or CLI used; which may cause problems (other BZs reflects that). This "REST-pure" use case is just working fine.