I don't think I'm alone in this quandry. In 2001, I switched from a feww version of Discus to vBulletin. At the time, there was no converter availabel (and as far as I know, there still isn't one), so many old messages had to be copied and pasted to the new board by hand.
The results aren't pretty; there's just one example.
So, seeing at there's now autocomplete with other search for user functions, would it be possible to incorporate this into a plugin that would allow someone to change a poster name just as easily as they could change a thread title?