Vinrouge, the pension calculator does a good job of answering your questions. I was led to believe that AFPS75 is a final salary pension; so it's based on your final salary. You'll stop accruing benefits, but those frozen benefits will be applied to your age 60 final PAS salary, not the salary you were on in 2015.
If you're my sort of age (mid-late 30s with a PAS offer), AFPS15 is extremely generous. The lower accrual rate is normally offset by it being a career average scheme, but if your career average is based on starting AFPS15 on top rate Flt Lt and ending on top rate PAS, it's obviously pretty high.
The pension calculator allows you to say when you will transfer to PAS in the future, and what level you think you'll enter it, and works things out from that.