I had assumed alphanumeric route indicators would only be used where there are many possible routes available - which isn't the case here. Why not just a position 4 'feather'?
Especially when the signal being replaced had just this! That's what I asked myself earlier today and didn't come up with an answer. I can only guess that if theatre/alphanumeric route indicators are in some way "preferred" nowadays, then this could be a reason. Personally I'd think that a position 4 "feather" would be clearer and less ambiguous and, after all, what does "G" signify, going back to the very first question, because it's not immediately obvious although with route knowledge I guess it should be.
If MS4388 were similarly changed, this is the signal on the viaduct in the up direction before Cheadle Hulme, protected by flashing yellows and, formerly, just approach-released, I'd be surprised. Position 1 means Macclesfield. So I find the Stechford change a little hard to understand also.