If you were eligible for both, you have to pick one. You can’t use both. I personally used the VA’s Veteran Readiness and Employment (VR&E) program (Chapter 31, formerly known as Voc Rehab). They paid for my bachelors and masters. I’m currently using the remainder of my Post 9/11 G.I. Bill for my doctorates. To qualify for VR&E have to have a disability rating of at least 10%. The program covered all tuition (even Georgetown for my masters, which was $4K a class), paid for new laptop, printer, great parking pass for the state school I went for my undergrad, etc. It also pays you a “Subsistence Allowance” which is pretty much BAH.
https://www.benefits.va.gov/vocrehab/
If you contributed the original $1,200 into the MGIB and convert to the Post 9/11 G.I. Bill, as long as you’re receiving housing allowance once your Post 9/11 runs out, then you’ll receive that $1,200 back.