Very often I recieve questions about ISA uprading. Here you find a summary of the various upgrade scenarios for ISA Editions:

1. ISA 2000 Standard Edition to ISA 2000 Enterprise Edition

Yes. Upgrade possible.
(Source: http://www.microsoft.com/technet/prodtechnol/isa/2000/deploy/isastnin.mspx)

Enterprise Edition Upgrade Process:

You can perform the following steps to upgrade from ISA Server, Standard Edition to ISA Server, Enterprise Edition:

  1. Although the policy will be preserved when you upgrade, it is recommended that, as an extra pre Caution, you back up the ISA Server policy.
  2. Run Setup from the ISA Server, Enterprise Edition CD-ROM. The setup process is very similar to the installation process for ISA Server, Standard Edition. The installation process sets up the ISA Server computer as a stand-alone server.
     
  3. Run the ISA Server Enterprise Initialization program. For more information on initializing the enterprise, see the ISA Server, Enterprise Edition Help.
  4. Promote the stand-alone server to an array member. For more information on promoting a server, see the ISA Server, Enterprise Edition Help.
     

2. ISA 2000 xx Edition to ISA 2004 Standard Edition

See the following table:

   From       Supported
ISA 2000 SE    Yes. ISA 2000 should be running SP1
ISA 2000 EE    No.

(Source: http://www.microsoft.com/technet/prodtechnol/isa/2004/plan/faq-installupgrade.mspx)

Can I upgrade from ISA Server 2000 Enterprise Edition to ISA Server 2004 Standard Edition?
No, an upgrade path is only supported from ISA Server 2000 Standard Edition running at least Service Pack 1.

3. ISA xxxx Edition to ISA 2004 Enterprise Edition

See the following table:

   From       Supported
ISA 2000 SE    No direct migration
ISA 2000 EE    In-place upgrade
ISA 2004 SE    Via Export/Import
ISA 2004 EE Beta    Via Export/Import