Pretty sure Tobira is set up so that with the exception of a few kanji, you should be able to pick up from any of the popular textbooks (Nakama, Genki, Minna no Nihongo, etc). I'm fairly certain that was a key decision in their design philosophy. Obviously it's going to be a little rougher than the transition to IA would be, but that's more down to the change in the way you learn than anything. I bought Tobira, and though I haven't started it yet, flicking through, I felt pretty confident about handling it after I finish Genki. Could always buy both and go through them in parallel if you have money to burn.
Edited: 2014-03-20, 7:52 am