You don't necessarily need that. But if you implement it, the better for us.
As SMF would reside in a completely new folder ( not in ../www/cgi-bin/, but e.g. ../www/vhosts/.../htdocs/forum/ ), you can install SMF in parallel, import the yabb data (the converter works just fine) and all is good (well 95% or so).
As the converter doesn't touch links in the posts, you just keep the old yabb install as it is right now and all the links will still work.
Then you could do some referrer checks in .htaccess to make sure that incoming traffic that tries to get to the old forum directly is forwarded to SMF (via the database lookups or maybe not), and to get to linked posts traffic originating from SMF back to the old forum would not be redirected.
I'll donate 50$ if this gets done before Xmas.