Vesuvan Shapeshifter says:
As Vesuvan Shapeshifter (...) is turned face up, you may choose another creature on the battlefield. If you do, until Vesuvan Shapeshifter is turned face down, it becomes a copy of that creature (...)
My understanding is that due to the word "as", this is a replacement effect. There is a ruling for Vesuvan Shapeshifter on gatherer that says:
(...) any “[when/as] [this creature] is turned face up” abilities of the chosen creature will work if Vesuvan Shapeshifter is turned face up.
For triggered abilities such as "when this creature is turned face up..." this makes sense to me. The Shapeshifter is turned face up as a copy of the creature so the triggered ability sees that, due to 614.6.
I don't understand why replacement effects such as "as this creature is turned face up..." would also work. My understanding is that once an event has been replaced the game moves forward, and replacement effects cannot effect past events per rule 614.4.
So from my understanding, the Shapeshifter is turned face up as a special action, its controller chooses another creature, the "turn Vesuvan Shapeshifter face up" special action is replaced with a "turn Copied Creature face up except it has Vesuvan Shapeshifter's upkeep triggered ability" special action and the special action finishes. Vesuvan Shapeshifter would then have the replacement effect of the copied card, but the effect would never have had a chance to do anything because the game had already moved on.
Could someone explain why that is incorrect?