https://en.wikipedia.org/wiki/Developed_country
Developed vs developing vs undeveloped are pretty well defined. They are relative terms. So sure you could in theory move from developed to developing, that kind of thing happens very rarely though.
I do agree that "developing" implies progression towards developed, so a more accurate term would be something like "partially developed" but its clunkier. Overall though I think they are far more accurate terms than the alternatives.