50 documents is actually a rather low number... How long does it take to change a field?
Mass transaction would work as long as the field is available on the list there, but the problem is that it can only be used to change one value to another. E.g. you can change Sunday to Saturday, but there is no option in one step to change <everything that is not Saturday> to Saturday. You'd have to run it several times (once per each combination).