We are in the process of moving our SVN repositories from one machine to another one, and with it will come a new domain name for the new repo. The problem is, that within
I'd use SvnDumpTool for this. It has exactly what you're looking for:
svndumptool transform-prop svn:externals "(\S*) (|-r ?\d* ?)http://oldserver.net(/\S*)" "\2\3 \1" source.dumpfile source-fixed-externals.dumpfile
This fixes up each external to the subversion 1.5 format, and uses relative URLs.
So svn:externals like:
external/libraryA svn://oldserver.net/repo/libraryA
become:
/repo/libraryA external/libraryA
using server root relative URLs.