This is quite a complex ask for a forum, but here's what I can tell you.
Firstly, when downgrading from 10.1 to 9.0, you are hitting 2 PAN-OS releases which are end of life and end of support. Those versions are 10.0 and 9.0. If something goes wrong when you reach either of those versions, it may be difficult for Support to help you, and you may simply be asked to factory reset and upgrade back to a supported release.
While I don't know your reasons for doing this, I can't recommend this course of action. If you must downgrade, I would advise getting to 9.1 and no lower.
So about the actual downgrades.
1) I believe you would need to downgrade step by step, 1 release at a time. 10.1 -> 10.0 -> 9.1 -> 9.0.
2) Usually when downgrading, you should select a config which was saved on the version you are downgrading to. It's unlikely your config can be used for the downgrades. If you don't or can't do this, you will very likely see autocommit failures and have to manually delete some things from the config on most downgrades.
3) There are a lot of things to consider when downgrading due to different functionality, especially if you've actually been using 10.1:
Upgrade/Downgrade Considerations (paloaltonetworks.com) (10.1 -> 10.0)
Upgrade/Downgrade Considerations (paloaltonetworks.com) (10.0 -> 9.1)
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