He changed from having a slow tree trunk back there at QB to a fast mobile QB.
Not fully true. OSU has had good success with both 'fast' 'mobile' QBs and slow ones.
Since Tress came on, OSU has had 6 QBs - Bellasari (2001), Krenzel (2001-2003), Zwick (2004), Smith (2004-2006), Boeckman (2007-2008) and Pryor (2008-Current).
Of those, he won an NC with the slow tree trunk QB in Krenzel. Smith and Pryor are your typical dual-threat QBs (which he's done good with), but don't forget that the guy in the middle - Boeckman was in between your slow tree trunk QB and someone that could run a little, ala Roethlisberger...Albeit in a guy that regressed as his seasons went on. He took OSU to an NC with him as well, and did much better vs. LSU with him than with Smith against Florida in that NC.