In 1994, he said he would support the AWB. In the debate you cite, he said again that he would have supported it.
He does not support a new AWB. There’s a press release posted to FR, December 31, 2007, that details exactly what he supports, and it includes a detailed description of what he considers weapons of unusual lethality.
My point is that any AWB would be new, because there is none in effect now. The 1994 AWB expired. There is no doubt Romney would support the same AWB if Congress attempted to re-impose it.