Generally the VPN client uses the IP address as the Initiator ID. If you would like to change it, you could try and check if there are other options to do the same. Check the 'Connect using secure tunnel ID type' options.
I haven't tried it though. You could also consider assigning a static IP address to the interface if nothing else works.
If a Cisco VPN concentrator is used with a Router running DHCP, the tunnel can be created although the IP address of the outside interface is learnt dynamically.
http://www.cisco.com/warp/public/471/vpn3k_iosdhcp.html