Long time lurker - I have some insight into DL.

First off let's be clear - this does not devalue your skybonus certificate. While H and U have been "downgraded" one fare class, you forget that 2 fare classes (X and V) have been added at the bottom of the fare structure. T will no longer be the lowest fare class, usually reserved for sale fares - it will be replaced by X and V. Depending on the fare bands, the new T may actually have higher fares than the current U - you may have actually received an upgrade here.

On the F cabin front, A has always been the lowest buy up class. This is now replaced by G as the lowest buy up class. The new P and A will be used to further segment the large gaps between F,P,A that currently exist. G will still sit above RU so it's not like inventory isn't going to fall to the new G.