Having enough award miles on my account, I've always made a new booking and then canceled the old one. That way you get all miles and all money back (./. EUR 50). I think, you had that in mind.

It might be a different situation about changes. But as far as I know, it's only possible to change date and time on award bookings, not the routing. So I think, technically your flight was canceled and new booked as well.

Are you sure, that the new taxes were EUR 82 in total and not additional EUR 82 (EUR 282 = EUR 280 + EUR 82)?