Track user/mod "careers"

[Julchen]

Hi,

I'd like to display user titles depending on the dateline of the actual post i.e. when a user becomes a mod all older posts should still display "user" as title.

I considered using a small table like this one

- Julchen,<user group>,1.1.2000
- Julchen,<mod group>,1.1.2003
- Julchen,<user group>,1.1.2006

All posts from 1.1.2003 up to 31.12.2006 are to appear as written by "Julchen/Mod" and every post before or after these datelines shows "Julchen/User"

How and where do I include the necessary queries? Or is there already a plugin available which does this?

TIA

Michaela