GI vs Post 9/11

[big ben]

so ive been looking up the differences between the two, but am hesitant to switch from mgib to post 9/11 because i cannot figure out the army's motive for encouraging the switch. I see AFN commercials all the time telling us how and why to change over; but wouldnt the military want to keep people from switching to save money? i know it is designed to encourage enlistment, but you enlist under the mgib, so y bother getting people to change over if they already enlisted? does anyone have experience with the post 9/11 that could shed some light?