I am not sure that moving the island "in time" could work. The goal is to make it impossible to find the island again. They would have to "move it" to a time when it didn't exist. When would that be? When dinosaurs roamed?

Also, if they plan to have the island moved before Keamy gets there, they are not going to be able to do anything that dramatic. I would imagine the easiest thing to do would be to somehow manipulate the EM field so that the bearings they fly in on are all messed up and no longer lead them to the island. Bearing 305 could lead them into the mouth of a shark or something...that would be cool.