If you modify someone's member file, the topics they participated in are still named after the old account. You need some programming skills to change the member number in the topic file, and it's not so difficult if you know a thing or two about Perl.
Basically, create a script that opens all your topics (it's better to open about 200 at once, because if you work on too many topics at once the script will time out,) finds
A||(.*?)||(.+?)||(.+?)||(.+?)||(.*?)||(.+?)||(.*?)||(.+?)||00000000(.*?)
(Replace 00000000 with the old member number) and replaces the old member number with 00000001. That would make the topics he/she used to write with their new member number.
Then look at all the posts, find:
Z||(d{6})||(.+?)||(dd-dd-dddd)||(dd:dd SM)||(.*?)||(.+?)||(.+?)||(.+?)||(.+?)||(.+?)||00000000(.*?)and do the same as before.
This code was just made up so I have no idea whether or not it works.