I ended up booking FRA - NRT for 380,- € return which is a nice deal I think.

Before, I called EY in Germany (twice!) and asked if this specific fare is eligible for an upgrade and was told it was. I checked the fare rule and could not find anything which would prevent an upgrade either. I made the booking and called them to upgrade and all of a sudden I was told that the booking class is "T", which cannot be upgraded!...

Not too bad as I would have booked it anyway but still strange that EY does not seem to know their own rules and conditions. I somehow had a feeling that this would not work out and I was right - but still looking foward to go to JPN