The sync-from URL is stored as a revprop in the mirror repo. If on the machine with the mirror repository (my situation), use the svnlook tool to look and svnadmin to change:
[email protected][~]$ svnlook pg --revprop -r0 /path/to/mirror/repo svn:sync-from-url
svn+ssh://svn.abc.com:1234/svn/foo
You will see the URL of the repo to which your mirror is currently syncing. In the above example, the master repo URL ends with .../foo. It may not have a newline at the end, so your shell prompt may follow. Now you need to get that into a file as svnadmin uses a file for input to change revprops.
[email protected][~]$ svnlook pg --revprop -r0 /path/to/mirror/repo svn:sync-from-url > t.txt
Now edit t.txt to change the URL to the master repo. This can cause a newline to appear at the end of t.txt and result in obscure/meaningless error messages from svnsync. So get rid of it:
[email protected][~]$ cat t.txt | tr -d '\n' > t2.txt
Note that we now have t2.txt which is the sanitized file. Then use svnadmin to change the revprop to the contents of the just-edited and sanitized file:
[email protected][~]$ svnadmin setrevprop /path/to/mirror/repo -r0 svn:sync-from-url t2.txt
Note that t2.txt is used not t.txt. Finally, check your changes:
[email protected][~]$ svnlook pg --revprop -r0 /path/to/mirror/repo svn:sync-from-url
svn+ssh://svn.def.com:5678/svn/foo
You should see your new repo URL immediately followed by the shell prompt, with no newline. In the above example the URL ends in foo and is immediately followed by the shell prompt [email protected][~]$.