I just went through the process and got selected in the January board this year. You'll hve to get your branch manager to sign a conditional release. I'm not too spun up on the JAG Corps, but they shouldn't have too much pushback unless your branch is significantly understrength. I had to pester my branch managers just due to the fact that they are constantly busy and usually TDY.
You will likely have to do an age waiver and an Active Commissioned Federal Service Waiver. Both of these go through the Chief of Staff G1 Army for approval. I had to have an ACFS waiver signed.
The great thing about your resignation memo is that it is conditional, based on that you are selected. So if your waivers don't get approved or you don't get selected in the board, you don't resign your commission.
I spent 8 years as a commissioned officer in the Infantry. I finished Company Command and in my opinion, had a pretty good shot at making 20 years and retiring. It seemed like a race already though, get to 20. I easily see myself being a pilot and being in the army for as long as they have a job for me, because I will absolutely enjoy it more than my current disposition. Yeah, the pay cut will initially suck, but it will catch up in about 6 years. Worth doing something you know you will enjoy.
PM me if you'd like with any additional questions. I will be out this weekend, but generally check this forum weekly.