Found the solution: Either changing database connection in each crystal report *.rpt file or changing instance and user name of the database.
There are a couple of things that need to be done. There is a utility called CrystalReportsDSNSetter.exe which should be in the reports folder where all the reports are kept. You need to run this to tell the crystal reports where the database now is. It updates all the reports using parameters you enter by hand. CHECK the log file it produces to make sure any updates have worked. If they don't, time to call support. You can also do this with Crystal if installed on the server using the Set Location command.
Then in LDSD itself, there is a crystal component that needs to be configured to tell it how to login to the database for each report. It's a bit tedious, but you select all the reports one by one, configure, enter the username and password and save. Do the next one....
Now a couple of caveats. What I've outlined will vary a bit if you are using Oracle or if you are using a 64 bit machine for Portal (and also using Oracle). You may also have some custom reports in other folders, so I'd really practise with this on a dev enviornment first!
If you run serviceportal, you'll also need to run the configurator there to tell it the DB has moved and do a set location on the special area using to hold reports for service portal too. Same deal as above by running the CrystalReportsDSNSetter.exe.
Thanks for the quick detailed reply.
We run the serviceportal the servicedesk and we have a lot of custom reports in different folders. I guess it is easier to install a fresh unnamed instance of the database, so that there is no difference between the old and the new connection string.
Works for me!