In the old system, there were a lot of releases that had DiscIDs attached, but didn't have any CDs in the release events. When imported into NGS, these IDs were attached to releases that didn't have a medium assigned (including releases that previously had no release events at all). Changing the medium on these releases to something other than a CD requires an editor to remove all the DiscIDs, which would sever the link between that release and those DiscIDs if there was only one release created on import (although the previous relationship would still be documented in the Edit History).

Possible solutions:
It'd be great if users were alerted when they were removing a DiscID's last link to a release. A step up from that would be to check whether the DiscID matches another release in that release group, then ask the user if it should be assigned there instead, or ask the user if they want to create a clone of the release for the CD issue and move the DiscIDs to the clone.