Here's a good document
http://www.cisco.com/en/US/products/sw/secursw/ps2308/products_configuration_example09186a0080094680.shtml
Notice on one end (central pix):
isakmp key ******** address 0.0.0.0 netmask 0.0.0.0
That end will never have to change, only the remote when the central pix IP changes.