Okay, so, uh, I made a move action in which the player moved towards an enemy, which lasted 100 frames for an attack, but then I realized I needed to 240 frames instead of 100.
I changed the frame number, but now what is happening is that the player pops at the beginning of the action in a location which is too far and different from the last position, say, the last one was 288 pixels horizontally away from the target, and the one when it pops is 480 pixels horizontally away from the target.
Is there any way to solve it?

Yeah, I've noticed that. That's why I don't do long move times when they're moving to the enemy.
I'm not sure what to do about it, it seems like when you raise the time it takes to walk there, it offsets their position somewhere else...