Let’s say your old link was:
http://forum.joomla.org/index.php/topic,255762.0/topicseen.html
The new link would be:
http://forum.joomla.org/viewtopic.php?t=255762
Thanks to Brian and Kenneth for pondering on this one.
If anyone knows how to build an htaccess redirect to deal with variables, perhaps we can pass that on to the j.org forum team.