Hangartner is good, I'm not sure he goes anywhere, at least for right now. I could see the Bills going through this year with Bell, Wood, Hangartner, Butler, Walker, and if Hangartner isn't what they expected, giving the offseason to Wood to play OC and Levitre LG, or maybe at some point during the year. But I have to believe they aren't going to completely revamp the line 2 weeks before opening night on MNF against NE.
Also, don't assume Walker automatically goes back to RT and Butler to RG. If they like Butler's progress, it could easly be Bell, Levitre, Hangartner, Wood, Butler.