When you want to replace your old Proxy server with a new ISA server:
By submitting your personal information, you agree that TechTarget and its partners may contact you regarding relevant content, products and special offers.
Set up the new ISA server and set the Proxy server's default gateway to be the new ISA server. That way you can move your users over to the ISA server at your own pace. (And you don't have to do the whole process in one weekend!)