BACK UP YOUR WC before proceeding. When you upgrade to r991236 or later, single-DB (format 19) is enabled. If you're running trunk and using it for your WCs, you'll need to upgrade those WCs before svn will let you continue to use them.
Currently the upgrade from format 18 is performed by tools/dev/wc-ng/bump-to-19.py. CAUTION: It's just a hacky script and it has some very rough edges. In particular, it will assimilate any versioned directory, *including things like externals that it shouldn't touch*. If you have any externals, you must delete them (the populated directories, not the properties) before running the script, and then run "svn update" to get them back again afterwards. - Julian
