I'm looking at a phased migration from PIX to ASA and our ISP is currently routing a seperate public subnet through the PIX into our network. As the IP of the PIX pair is used in their routing tables, and the ASA's are being commisioned of different public IP's (it's too complex to just keep the same IP's for other reasons), I'd ideally like to be able to take the IP being used as the routing HOP and keep it on the ASA's as a secondary IP or such like, without having to liaise with the ISP to do a timed routing change, which is never fun. This would also give a nice bit of abstraction from hardware IP's and functional IP's. if this was an IP being used for a NAT or such then obviously that move would be simple, but as this is a routing hop, the NAT wouldn't make sense (would it?)
If this were IOS, i'd personally be looking at an HSRP IP, but on ASA I don't think this is possible, but hopefully someone might be able to prove me wrong.