How would I do this?

Easiest method is to make a broadcast which makes each object wait until a variable is lower than the objects y-position and then it goes to the front. Then run a loop to set the variable to 180 and step it down to -180 - steps of 5 or 10 may be OK rather than stepping 1 at a time. Add a wait 0 seconds and repeat if necessary.

Depends a bit on how much things are changing though. If only one or two objects are changing their layer there will be better ways to do this.

Here maybe try this:

Put this in each sprite you are using.

We can make 2* global variables, and set them to the y position of the sprite. Then, we can use an if statement to check every single frame whether or not one sprite is higher than the other.
*two is for demonstration purposes. Make as many variables as there are sprites/clones.
Kind of like this:

Thank you everyone for the help!