If you are testing Windows 2008 as a member server then there is no need to update the schema. If you are testing Windows 2008 as a domain controller then I would recommend the following:
1. Create a test domain that is Windows 2008 level functionality. That way you don't have to modify your production schema for testing.
2. If you choose to test Windows 2008 as a DC but DO NOT create a separate domain/environment for it, then I highly recommend that you not use a time-limited trial of the software. One day you'll be very unhappy when that trial period expires and you can't get into the DC anymore.
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CompTIA A+, Network+, Server+, Security+
MCSE:Security 2003
MCITP:Enterprise Administrator