As far as I know, no threads version has that feature built in, whereby you can edit the markup/HTML selection.
So you'll need to run an SQL query from your admin panel.
Here it is...
UPDATE w3t_Posts
SET B_Convert = "both"
WHERE B_MAIN = ID# OF FIRST POST IN THREAD <--- fill in this number, that's it
For example...
UPDATE w3t_Posts
SET B_Convert = "both"
WHERE B_MAIN = 2902
If any HTML code is already posted in the thread, it won't automatically convert it after doing this btw, unless the person goes back and edits new HTML into their post.
Well that's the theory