Righty-o then. Docs don't appear to be comprehensive and/or entirely up to date. Yay!

Anyway, line breaks are coded as:
<br/>\n
Which needs a simple replace with:
<br/>
Bold is done as:
<B><s>[b]</s>bold text here<e>[/b]</e></B>
Italic is:
<I><s>[i]</s>italic text here<e>[/i]</e></I>
Bold nested inside italic is:
<I><s>[i]</s><B><s>[b]</s>bold italic text here<e>[/b]</e></B><e>[/i]</e></I>
Standard smileys use an uppercase E tag around the smiley code:
<E>:lol:</E>
BBC colours are:
<COLOR color=\"red\"><s>[color=red]</s>red text here<e>[/color]</e></COLOR>
Code boxes are:
<CODE><s>[code]</s>00 00 02 00 00 00 00 00 00 00 00 00 00 04 00 04\n20 xx <e>[/ code]</e></CODE>
With the line breaks inside code tags apparently being done with \n rather than the standard HTML br tag. Which is a trap for young players. It means you can't just delete all instances of \n in the posts table. You'd only want to delete the ones that are directly preceded by a br tag.
Inline attachments are:
<ATTACHMENT filename=\"massive_lolcat.jpg\" index=\"0\"><s>[attachment=0]</s>massive_lolcat.jpg<e>[/attachment]</e></ATTACHMENT>
Most of these are pretty easy to deal with. The tricky ones are going to be in-post links, which are going to have varying url's in the middle of the gobbledegook.
<URL url=\"http://www.example.com/about.htm\"><s>[url=http://www.example.com/about.htm]</s>this page<e>[/url]</e></URL>
That's going to require a wildcard search to knock out the superfluous url tag in front of the s tag.
And then there's this weird shiznit:
<URL url=\"http://example.com/forums/viewtopic.php?f=67&t=811&start=170#p42039\"><LINK_TEXT text=\"viewtopic.php?f=67&t=811&start=170#p42039\">http://example.com/forums/viewtopic.php?f=67&t=811&start=170#p42039</LINK_TEXT></URL>
Not sure WTF that is supposed to be.
