This is actually not that hard, but might be time consuming, depending on your repository size. Anyhow, the bdb database works fine, but in my case, when moving between Debian releases bdb breaks due to compatibility, so fsfs is the prefered way nowadays. if you don´t want to screw around with different bdb releases.
- svnadmin dump /PATHTOYOURREPO > backup.dump
- mv /PATHTOYOURREPO /PATHTOYOURREPO.bak
- svnadmin create --fs-type fsfs /PATHTOYOURREPO
- svnadmin load /PATHTOYOURREPO < backup.dump