I've recently had to change my Subversion host of choice. The said host fell out of my control and, while I completely trust the new admin, I cannot rely on that server staying indefinitely available.
Fortunately, I now have access to my own (shared) server, for which I also have an SSH access. I'm quite a nitpicker, though, and wanted to be able to use my old user name on the new server (mainly to keep consitency amongst revisions). Of course, since it's a shared server, I cannot simply create a new system user. Documentation is pretty hard to find regarding this, and it is split between multiple locations. Here is a concrete example.