We are currently using Kopia 0.7.3 and would like to upgrade to the latest version (0.9.5). I see that there have been changes in repository format and there are instructions on how to upgrade old repo format to the new one. But it is not clear to me whether this upgrade is mandatory or if latest Kopia version can continue to work with old format (albeit without the optimizations introduced in the new format).
The update is, for all intents and purposes, mandatory, as laid out here, specifically:
The optimistic plan is:
v0.9 supports new repository format by default, old format can be still be enabled using flags
v0.10 supports old and new format, will be prompting legacy users to migrate to the new format
v0.11 supports new repository format only, old index format not supported
Assuming that there are no significant issues with the repository format, it will be made mandatory in the future and backwards compatibility is not guaranteed, so you are better off upgrading.