Is there a list of db schema changes for different releases?

[Covance Genomics Lab System Support]

I am testing upgrading from jira 6.3.9 to 7.0 to 7.13 and then to 8.15.  We copied our production db to a test version and I will work with that. If I do upgrades will it prevent me from downgrading back to an earlier version?     For example if I upgrade my test instance from 6.3.9 to 7.0.11 and then decide to run 6.3.9 again, will it work? Or does the upgrade alter the db schemas too much?   

Is there a list of the schema changes per major release?

[Nic Brough -Adaptavist-]

There is no downgrade path.  It requires a LOT of code to downgrade each version, there's virtually no use for it, and it is immensely more simple to take a backup before upgrade and revert to that if you need it.  The only real use for that is if someone doesn't test their upgrade process properly, misses something and then needs to revert because there is something broken after they've entered more data.

For the same reasons, the documentation is pretty sparse, best you can do is diff the docs and hope you can work out what you'd need to do to downgrade.