If I used 16 months of my post 9/11 GI Bill to complete my undergrad. Do I need to reapply to use my remaining 20 months for my masters?

Hey 1 LT Boeh,
I'm an Army Education Counselor, you'll have to fill out and send in to the VA the VA FORM 22-1995 (Request for change of program or place of training).

No you don't. I used half of my GI Bill for undergrad and the other half for my master's degree. I would recommend doing research on the graduate school you plan to attend and reach out to the Student of Veterans Affairs office if in the event you need to follow through with some paperwork. For the most part you should be fine.