You have the option to convert any unused MGIB to the Post-9/11 GI Bill—you cannot use the same period of active duty to qualify for the Post-911/GI Bill and the MGIB, contact the DVA or your education office.
To convert after the first year of AD, you need to apply for the Post-9/11 GI Bill by completing an application with the DVA. If member converts to Post-9/11 GI Bill after attaining eligibility and before the entire $1200 is reduced, they may elect to have deductions stopped. Refund will come when member exhausts Chapter 33 benefits.
Regardless of when you convert, a proportional amount of the initial $1,200 MGIB pay reduction will be refunded in your last monthly housing stipend payment, if you attend ¾ time or more and do not attend exclusively online.