Sure, it sounds fine when someone with Wade Phillips's experience says the change won't be much, but as you point out, Greene did suffer in the move
Now. there were some things going on, for one, Greene stood up a lot as a DE in 1991 and he also had to move from right to left and back to LBer due to injuries to others, but it was not a success. Then, the next year, Knox had Greene in the 4-3 again but it was a bit of a different role on run downs.
The closest thing to the DOnald situation is Jerrell Casey, he was an up an comer at 3-tech and they went to the Pittsburgh style one-gap 3-4 when Ray Horton and then LeBeau came in. He had 10 sacks final year of Jerry Gray's D---which is very similar to Williams/Fisher and in the 3-4 as a 3-tech the last three years he's averaged about 6. Now he's been good, sacks are not only thing, but still, the way things go is not always how they are planned...Donald, IMO, is better than Casey, but in terms of sacks, there is a possibility that his production will go down. But, if Rams win, no one will care, If our SAM and WILL players get 10 sacks each and the inside pressure caused by Donald helps them, then it's all and good.
Just saying these moves are sometimes easier said than done
