We had originally planned to develop an upgrade path from Zorin OS 12 to 15, however there wasn’t enough time during the last development cycle to finish and test this feature. Zorin OS 15 was a rather ambitious release with a lot of new improvements which caused us many delays. Adding this would have delayed it even further, so we decided to not spread ourselves too thin.
However, we’re planning to enable upgrades between Zorin OS 15 and 16, as this feature is now one of the most high-priority items for this development cycle.
Moving away from the Ubuntu repositories isn’t likely to be of much benefit to the Zorin OS project (in balance against the costs of doing so) and is currently not a priority for us, so I doubt we will make this move in the foreseeable future. However, if anything happens to the upstream Ubuntu repositories, we should technically be able to replace them with either Debian’s or entirely our own ones in a way that would feel like a regular software update from the end-user perspective. We already operate our own repositories on our own servers, so most of the infrastructure is there today. Nevertheless, as this would detract significant resources away from developing and improving Zorin OS from a user experience standpoint, I don’t think we will need to do this anytime soon.