Well I believe that Masters is just a sub-catergory and in most instances you can cross over. Usually you must be over 35 (some organizations I believe it is 40), so you can compete in the Masters division against other women your age and then compete in the open catergory. There is also a Master's Nationals where you can qualify for your Pro-Card competing against other older women as opposed to the 20 year olds.
You aren't actually changing your status, just expanding the catergories you compete in. I'm guessing some women are slightly worried about competing against much younger women and want to feel more comfortable, also with the dramatic rise in Figure contestants it is a way to give more appreciation to those who may get lost in the masses.