Manually changing all of those external properties sounds tedious. Have you looked at the new functionality for externals added in Subversion 1.5?
Subversion 1.5 takes a huge step in relieving these frustrations. As mentioned earlier, the URLs used in the new externals definition format can be relative, and Subversion provides syntax magic for specifying multiple flavors of URL relativity.
../
Relative to the URL of the directory on which the svn:externals property is set
^/
Relative to the root of the repository in which the svn:externals property is versioned
//
Relative to the scheme of the URL of the directory on which the svn:externals property is set
/
Relative to the root URL of the server on which the svn:externals property is versioned
Maybe one of those would help? I guess it depends on exactly how you are branching and what your repository structure looks like.