Not applying a standard universally (a), or changing the standard itself as you go (b), leads to either an inconsistent adherence to the standard (a) or adherence to an inconsistent standard (b).
To forgo the ambiguity between these two very similar options, I would simply call that inconsistency.
Not staying the same throughout.
‘the quality of the material was often inconsistent’
1.1. Acting at variance with one's own principles or former behaviour.
‘parents can become inconsistent and lacking in control over their children’
(inconsistent with) Not compatible or in keeping with.
‘he had done nothing inconsistent with his morality’
Although I understand your reasoning for drawing analogies between "code swing" and "mood swing", I would also argue that it's not the code that swings back and forth, but rather the adherence to the coding standard.
So it would be more accurate to call it "coding standard adherence swing". In the case where you adjust the coding standard, it's "coding standard swing". Or if you consider not adhering to coding standards to be an issue of code quality, you can argue that it's also "code quality swing".
I can't tell you it's not allowed to coin a new phrase, but I can tell you that I would not have understood what you meant by any of these neologisms, and I'm a software developer who very much gets annoyed at half-assed adherence to coding standards.
So in the interest of clear communication, I would suggest you avoid this particular neologism. It's not clear what you mean, and you'll have to end up explaining it to people anyway, thus defeating the purpose.