You can't really change the registration point. But you can
achieve the same effect.
You have two options for that example
a. You can draw your square in a child clip of square_mc.
Then you just move the childclip's position to have the effect of
changing the registration point of the parent.
b. you can set up a function that draws the square using the
api like you have done inside square_mc. You can move to -50,-50
and draw from there after using clear() to erase the previous
So you can 'adjust' it by redrawing from a different start