GroundBound,
IMVHO you are spot on! Following initial contact report by Flipflop 123 (great callsign! cheers Kwimper) correct R/T by the second ATC (that presumably still works with the level by restriction) in my mind should be "after GWC descend FL120".
Same with a lateral clearance?
“Flipflop 123, cleared dir ABC”
“Direct ABC, thanks, Flipflop 123”
“Flipflop 123, contact SomebodyElse on 123.45”
“123.45, seeya, Fliflop 123”
While on the way to ABC:
“SomebodyElse, Fliflop 123 hdg dir ABC”
“Flipflop 123 cleared dir DEF”
Now that clearly replaces the previous hdg clearance. If that was not the intention then the new instruction should be: “Fliflop 123 roger, after ABC fly dir DEF”
“…and that’s what I think about that.”