Switching ISPs without loss of service - Multiple Gateways

Here's the situation:

I have a windows 2003 server with ISA 2004 on it.

I have my DSL internet into a router, then to the ISA server.
I'm switching to a local Cable company.

So, now DSL and Cable go into a switch and then into the ISA server.
I have 5 static IPs with each provider.

If i set the gateway for the network card in the server to the DSL gateway, my DSL WAN IPs work. The cable IPs don't.
If i set the gateway for the network card in the server to the Cable gateway, my Cable WAN IPs work. The DSL IPs don't.

I don't want to just switch my DNS to the new Cable IPs, because I'll likely loose some email in the transition.

Any ideas?  Ideally, i'd like both sets of IPs working so i can switch seamlessly.

Thanks in advance,

James