Reading up a little on regular expressions helped me write this solution to my .htaccess file:
RewriteEngine on RewriteBase / RewriteRule ^2003([0-9][0-9]) archives/2003/$1/ [R]
There’s a similar
RewriteRule for each year of archives. So now any old-style permalink pointing to something like
/YYYYMM#123456789 will redirect to
/archives/YYYY/MM/ which is that month’s archive. Not perfect yet, but at least a heck of a lot better.
There are also some additional links to helpful MT link conversion methods in the comments to the previous post — for those having to deal with the same scenario.