Policy server version needs to always match Policy store version. There isn’t any exception to this.
Why would you want to use old policy store as failover store? Could you not just create another instance of new policy store as failover?
If you want to run old and new policy store in parallel , you will need to have corresponding version of Policy server pointing to it and use common key store. But with this approach as there wont be any policy store synchronization between old and new policy store , this should be used only temporarily during migration.