Thanks for answering my questions.

I think my questions weren't clear enough.

I'm not interested in short-haul "business" class on LH. I plan to stay a few more hours in FRA, so I need to get on a later flight. Based on AC rules same day changes are free in Y. In P they incur a fee. So if I were to change the FRA-IST leg to a later flight on the same day and the FRA-IST leg is issued in Y, I'll be OK. If it's issued in P I'll need to pay.

What I don't want to happen is AC charging me a change fee, saying the whole reservation is issued in P, and then credit 150% miles for the FRA-IST leg instead of 200% claiming each leg is issued in a different fare class. This is confusing because on my AC reservation page, it says "business class lowest" for the FRA-IST leg, but next to it it also says "booking class" Y.

Before posting on FT I called AC twice to make sure I get a consistent answer. No such luck. The two reps I reached gave me different answers: P all the way through, so yes you need to pay a change fee; P and Y, so no you don't need to pay a change fee.

I didn't even bring up the mileage issue...