Short version of (correct) tzaman answer will be (for fresh SVN)
svn switch ^/branches/v1p2p3
--relocate
switch is deprecated anyway, when it needed you'll have to use svn relocate
command
Instead of creating snapshot-branch (ReadOnly) you can use tags (conventional RO labels for history)
On Windows, the caret character (^
) must be escaped:
svn switch ^^/branches/v1p2p3