My recommendation, considering the 2 year support policy and the eligibility for upgrade price (up to two prior versions), is to upgrade every two major versions. In your case I would skip PDM 9 and wait for the next major version.
Windows Server 2003 Guest is supported in all these PDM major versions and I don't see why it would be dropped in the next major version, here's the list of supported OSs in PDM 9: http://www.parallels.com/products/desktop/#tab4
PDM 8 (in it's latest minor version) supports both ML and Mavericks.
What I would do: Backup everything first, make sure I was running the latest version of PDM 8 by checking for updates, then update to Mavericks, if you get any trouble running PDM 8 after upgrading to Mavericks reinstall PDM 8 (again, using the very latest version of 8 available, you can download it from the site, links in my signature).When Yosemite comes out wait to see what Parallels has to say about Yosemite support and if there's a major version of PDM coming. If what happened last year holds true, PDM 8 might not run on Yosemite and won't be updated in order to be compatible (end of 2 year support cycle), in that case, if you want to upgrade to Yosemite, THEN you should consider upgrading Parallels to the latest major version available.
Last edited: Jun 19, 2014