If an SVN command is used on any file, it will automatically upgrade the directory to the most recent version of SVN. This will result in the error: This client is too old to work with working copy
The way to fix this is either to upgrade the client, or to downgrade the directory to a previous version. This can be accomplished using the script change-svn-wc-format.py from http://subversion.apache.org/faq.html#working-copy-format-change
To use this script: change-svn-wc-format directory version, where version is the SVN version (1.4 or 1.5). For more help, type: change-svn-wc-format --help