"Sometimes the working copy metadata format changes incompatibly between minor releases. For example, say you have a working copy created with Subversion 1.4.4, but one day you decide to try out Subversion 1.5.0. Afterwards, you attempt to switch back to 1.4.4, but it doesn't work — it just gives the above error.
This is because 1.5.0 upgraded your working copy format to support some new features (in this case, changelists, the keep-local flag, and variable-depth directories). Although 1.4.4 doesn't know anything about these new features, it can at least recognize that the working copy format has been upgraded to something higher than it can handle.
1.5.0 upgraded the working copy for a good reason: it realizes that 1.4.4 does not know about these new features, and that if 1.4.4 were to meddle with the working copy metadata now, important information might be lost, possibly causing corruption (see issue #2961, for example).
But this automatic upgrade behavior can be annoying, if you just want to try out a new release of Subversion without installing it permanently. For this reason, we distribute a script that can downgrade working copies when doing so is safe:
Run that script with the "--help" option to see how to use it. As future versions of Subversion are released, we will try to keep this FAQ entry up-to-date with potential downgrade scenarios and their implications." subversion.tigris.org