From what I understand, a team can only wear a "special", "throwback" uniform twice a season. Any more and the league would consider it an alternate uniform, and you would need league approval, a year in advance, and there are rules as to how often you need to wear them.
I do not know if the Bills went through any of those procedures, or if they plan on doing that. My belief is that public opinion will decide that.
Also, from what I understand, an alternate uniform, approved by the league as such, would also have to abide by rules on size and clarity of numbering, location of side numbering, size and clarity of names on the back, consistency with the main uniforms, and I'm not sure whether or whether not the throwbacks conform to those rules now.