I've seen tons of questions around if you can use various components of SRS 2000 with SSRS 2005 (and vice-versa), so I'm attempting to document them all in once place. This will hopefully become a good FAQ on the subject...feel free to ask/add more to this list via the comments section
Q. Can you use the 2005 Report Viewer controls against SSRS 2000?
A. No, you can't.
Q. Can you use a 2000 RDL with the 2005 Report Viewer control?
A. No, you must first upgrade the report to 2005.
Q. How do I upgrade a 2000 report?
A. To upgrade an SSRS 2000 report to 2005, open it with the 2005 designer, or upload it to SSRS 2005 and it will be dynamically updated.
Q. Can you publish a report from VS 2005 to SSRS 2000?
A. No, you can't the RDL schema has been updated since 2000, and your 2000 server won't understand the 2005 schema.
Q. Can I use VS 2005 to create 2000 reports?
A. No, we generate 2005 RDL, which SSRS 2000 won't understand.
Q. Can I run SSRS 2000 and 2005 side-by-side?
A. Yes, you can. You must install 2000 as the default instance and 2005 as a named instance. See BOL for more information.
Q. Can I publish my SSRS 2000 RDL document to 2005?
A. Yes, you can. You can upload the document via Report Manager, publish directly via Report Designer 2000, or use the SOAP API.
Q. Can you install the 2000 and 2005 designer on the same machine?
A. I haven't tried it myself, but supposedly, yes...this should work.
Q. Can I host the 2000 Report Designer inside VS 2005?
A. No, you can't.
Q. I want my SSRS 2005 instance's ReportServer (system) database to be hosted on SQL 2000. Is this possible?
A. Yes, it is.
Update: I've added a seperate post on this:
Q. I published my 2000 report to SSRS 2005 via Report Manager. Can I export it and use it in SSRS 2000 again?
A. Yes, you can. (Thanks to Brian Hartman). If you use Report Manager and choose "edit" you get the original RDL you published right back. However, if you opened the 2000 report in VS 2005 and *then* published it to SSRS 2005 via the designer, you won't be able to "roll back", as VS 2005 upgrades the RDL schema.
Q. I have SSRS 2005 SP1 installed and I've deployed my reports to this machine. Can I re-publish these reports to a SSRS 2005 RTM box?
A. Yes, SSRS 2005 reports can be moved back and forth between RTM and SP1 instances with no problems.