Declarer has AKQ109 in dummy and 87 in hand. Declarer crosses to AS and then says 'Play from the top' referring to spade suit.
Lefty shows out on second spade. Can Declarer now change her instruction?
Yes, This is in the White Book.
Suppose declarer instructs dummy to “run the clubs”. Declarer may change this instruction at a later trick, and a card from dummy may be changed until declarer’s RHO plays to the trick. At this point the card becomes played.
When I play with a pickup partner, I find they fairly commonly ask me to run a suit when I'm dummy. I wait for them to motion for me to continue after every trick anyway, mostly because of this ruling.
In general I don't think it's legal for declarer to give dummy instructions that last more than a trick (although it is very common). After all, declarer's meant to be designating a card, not a strategy. So it's proper for dummy to wait for instructions whenever it's their turn.
On the rare occasions a partner asks me to "run" a suit, I sometimes noticeably hesitate because I am so used to waiting for my next instruction and I have such a short attention span when I'm dummy that I have forgotten the previous instruction to run the suit. I solve it by strongly encouraging my partners not to instruct me to "run" things, an irregular practice that is rightly "deprecated" by the WBF LC, according to the White Book.