It would be nice if Tom could say something about this. I've installed ISA 2004 beta into production and it's working great and I was confident that when the final version came out I could intall it over the beta without reconfiguration tasks.
I'd also like to know what happens when the beta evalutation time expires, will it stop working?
Is there an official MS newsgroup I can ask thee questions to?
At this time the recommendation is that you DO NOT install the product on a production network. There is no committment that the current policies will work in future versions. We may be able to come up with some hacks, or we may not, but I would *not* assume that you'll be able to back up your current config and apply it to the final versions.
Its hard to tell now if its going to upgrade. When the time comes and you have ISA 2004 on a box try exporting your setting to a .XML file for a backup. Install the final release and restore from the backup. This works very well.