This article explains the possible ways to downgrade a PAA server.

There is NO possible way to Downgrade an existing PAA server. Only possible ways are mentioned below:

-In case if you have a PAA backup then you can restore it after fresh install on the same release only.
please refer below URL for reference. 
https://www.juniper.net/documentation/us/en/software/active-assurance4.3/paa-upgrade/paa-operations/topics/concept/restore-data.html

- If during upgrade, you wish to have a rollback plan, then only way is to redeploy a new Control Center server and restore the PAA Backup taken during the upgrade steps.
Refer - https://www.juniper.net/documentation/us/en/software/active-assurance4.2/paa-upgrade/topics/concept/rollback-failed-upgrade.html