As is usually questioned on this forum, its the old instruction superceeded by another instruction, therefore the second replacing the first unless reiterated. Your situation sounds like an instruction issued and then a tx of control/comms takes place, so if you check in on the new frequency and state the speed restriction (as you should) then its up to the next controller to reissue it if they change the current terms unless you simply say "speed 250kts" and miss out the "when level" bit which I've known to happen. The other thing to think about is the SLP, had you passed it or would you pass it while descending FL90, then its even more complicated! If in doubt, ask. More of us controllers are aware of the ambiguity of some instructions so will take the measures needed to avoid this, but not all of us are up to scratch just yet
I've tried avoiding using this phrase, and am tending towards "descend FL150 lvl Tiger, make the restriction and when able speed 250 kts", it works well most of the time
