And does this document help? I found it searching on "SQL 2005 licensing structure" but couldn't get an actual link (nor find it on Microsoft's website even though opening it took me directly to download.microsoft.com).

EDIT: and then there's this link here. Which is also vague, but sounds like you can actually do this.

EDIT 2: This final link seems to indicate you need a separate Standard license key in order to "downgrade".