How safe is it in general to upgrade within the 3.X version range, especially in terms of the PostgreSQL database schema? If the schema would potentially change, would supertokens guarantee that migrations and adjustments are correctly applied upon startup or would it require manual work? And/or would something like this considered a 3.x -> 4.x change?

hey! You should checkout the changelog (https://github.com/supertokens/supertokens-core/blob/master/CHANGELOG.md) for this. If a new table is added and it's unreleated to existing tables, then nothing is required from your side other than restarting the core with the new version. If there is some manual data manipulation that's needed, it will be mentioned in the changelog along with how you can do that. Finally, if there is some manual change required from your side to the tables, it would be a 3.x -> 4.x change.