Any agreement is going to be for the Class B shares, and for them to remain Class B shares, with the removal of that clause about selling to non-Glazers triggering the conversion. That clause exists only so that if some of the siblings wanted out, that the rest stay in control.
Clearly any decision to sell requires all of the Glazers agreeing. Even if Joel and Avram retain a combined 20%, they're still in agreement on the sale (they would be selling some of their shares) and the change of that clause.