Hi Felix,
If you go to Administer -> Upgrade, there should be a link in there that shows which models that firmware supports. If R500 is there, then all you need to do is to downgrade your WAP to 100.x.x firmware. Firmware is firmware and downgrading/upgrading it should not be an issue. Once the WAP joins the ZD that firmware gets wiped and 9.8 version of it gets installed.
Hope this helps.
Cheers,
Alex