It's weird to me that people call 26 x 1 3/8" tires "650A" now. Most mechanics would have wondered what the hell you were talking about BITD.

Anyhow, +1, most likely they'll adjust to suit either. Many Japanese 80s touring bikes for the American market could switch between 27 and 700, and that's a slightly bigger difference. Worst case scenario would be new brakes required. Personally I'd try to keep it 650A for the coolness. Good tires are still available. It's not like S6.