Is there a way to change a SVN users username through the entire repository history?

Question

When my team first started out with SVN we all just used our first names when committing to the repository, however, now that our team has grown, we are running into issues

Answer 1

TortoiseSVN has excellent support for this functionality: within its Revision Log Dialog one can filter by author (even via regular expressions), select revisions from the filtered list as desired (usually all like in this question) and select 'Edit author' from the context menu.

The pre condition of having a pre-revprop-change hook in place as mentioned in jeroenhs answer does still apply, of course.

The processing is rather slow, but depending on ones needs this might still be much faster and/or more convenient then having to dump an entire repository and process these potentially huge dump file(s) with scripts.