The Bills aren't going to win the SB with Walker at LT, so you might as well try Bell to see what he's got. To paraphrase Buddy Ryan: We don't know if the young guy can do it, but we know that the old guy can't.
RT is a little trickier. Walker is an average starting RT. Maybe that's the best we've got and maybe that would be good enough to win a championship if the other line positions are strong, but we're paying him a lot of money to be average and I think that Butler should be given a shot to see if he can develop into a stronger player. So, if it's close between Walker and Bell in terms of performance, I would simply bench Walker for now and see how it goes. If it looks promising, then Walker is probably gone by next year.