Just as the other posts confirmed, you can not downgrade the ACS appliance, your best approach will be to keep a backup handy (and note which patch version you are on), once you upgrade to 5.4 and find that there is something wrong. You will have to reimage the unit, set the ip settings...dns...etc. Install the same patch you had before and then restore your backup.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...