36D, 120lbs, 5'1, will going from 170cc Natrelle silicone to 220cc low plus make a difference in cup size?

A lot of factors determine cup size- which also varies depending on bra company, your frame, existing breast tissue, measurements, etc.

170cc to 220cc is not likely to be a big change of a full cup size, and is likely not worth risking another surgery.

If your measurements allow, a larger increase such as 100cc (270cc implant) may give the results you are seeking and should be discussed further with your plastic surgeon.

Increasing volume will result in larger breasts but the amount or percentage of enhancement depends on a number of factors such as height, weight, amount of native breast tissue, base diameter. 50cc increase might be 1/2 cup size for an 'average' patient.

The 50 cc increase is about a third of a cup, so you will probably stay in the same size bra unless it fits tightly now.