When I go up to resign 15 months prior to my ets can I go from active duty to reserve or national guard?

When you are 6 months from ETS you can sign a reserves contract. It will go in effect the day after your ETS

As an enlisted servicmember you don’t resign. Officers do that when they resign their commissions. You can set it up so that when you ETS you transition fairly seamlessly into a reserve component unit. Talk to your career counselor.