I thought it would be smart to just take the old account file that I had saved (of course!), replace the master URL and copy it to the BOINC directory.

The effect was,that I suddenly had 2 Pirates projects, one for the old site and one for the new site, and the PC requested a new HostID on each contact so I ended up with about 20 host entries and way more WUs than I planned to request.

So it does not work like that, the new file needs the URL in the filename (account_pirates.spy-hill.net.xml).