Can a pituitary macro adenoma become micro adenoma?

Yes. Macro, large tumors, can 'shrink', with some medications, and, of course, by surgery.
It depends. It is in the eyes of the beholder..... Eyes of the reader of the pet or MRI of the brain. ..Changing the machine... Treatment... Certain endogenous hormone levels and changes need to be considered. If the lesion is borderline to start with... Readings could change... Consult with the doctor and the radiologist... Compare side by side the pictures and correlate clinically.