Coding changes?..
For lack of a better phrase, yeah.
Whos the programmer on this project?...
Evolutionary processes.
It would have to be a project..
Define the word "project" as you are using it in this context.
Scale to feather in one fell swoop is probably statistically impossible.. to belly laughing standards..
Perhaps, perhaps not (you sort of "forgot" to show your work leading to this conclusion of yours, concerning whether some simple structure vaguely featherlike enough to be marginally worthy of the name could arise through any sort of single mutational event from pre-existing integument structures), but even if so, I'm not sure what your point is, because they would not need to develop in "one fell swoop" as you put it.
So then.. a feather might develop from a scale in stages?..
The question would be why.. Why would scales develop into scales?.. Especially in stages.. A feather ANY feather is quite a complicated piece of work.. Not to speak of the organ(s) needed to support the growing and support of feathers.. and design work needed for various kinds of feathers.. for various functions and parts of the body..
After feathers design phase, variations could easily modifiy to further handle more unique tasks..