Hi Efffive,
Did you get an RMA unit on a newer version than what you're running on the existing peer? Or are you upgrading platforms?
In general, if you want to install an older LTM version, it's probably easiest (for 9.x) to reinstall via USB key. You can read the release notes for your 9.x version for details on how to do this.
The admin GUI should be available as in v9. Are you connecting over the management port or switch port? If the latter, make sure that port lockdown is configured for default or explicitly enabling 443. The management port should allow all ports by default. You could run a tcpdump to check if the traffic is making it to LTM (eth0 for mgmt or 0.0 for all switch ports).
Aaron