I've started getting questions like the following:
Given a report developed on SRS 2000
Given a report developed on SRS 2005
An RS 2000 RDL report can be published and run on an RS 2005 server. When you request the RDL from the report server for such a report the original, unmodified rdl is returned. You can use the RS 2000 report designer to publish to both an RS 2000 and an RS 2005 report server.
An RS 2005 RDL cannot be published to/edited in the RS 2000 server or tools.
When you use the RS 2005 report designer to edit an RS 2000 RDL, it is upgraded (you are prompted). Once upgraded, the RDL can no longer be edited in RS 2000 tools.