From MozillaZine Knowledge Base

Why not tell people to use the "Browse" button to pick the correct local directory, in each in the account server settings and in Local Folders, instead of copying over the contents of the old account folder to the new one?. For example. if the current local directory shown is pop.gmail-1.com, select the pop.gmail.com local directory. That worked for Mozilla Suite/Netscape 7.x if the account is removed and recreated. (ref.) Alice 04:54, 28 August 2007 (UTC)

Good idea - its simpler. I think a long time ago if you told it to use the exact same mail server / account type it would silently reuse the old account directory. However with 2.x it creates an account directory with a suffix. I'll change it. Tanstaafl 06:45, 28 August 2007 (UTC)

I tested removing and adding back a mail account in Thunderird 2 and you need to restart Thunderbird after browsing to the old local directory (same as you had to do in Mozilla Suite/NS7) before the old mail reappears. I added that to the article. Alice 13:26, 28 August 2007 (UTC)