How to convert from A4 to US Letter without the formatting messing up?

Hello, I am trying to convert an A4 document to a US Letter format. However, when I do so the formatting gets all messed up and crushes itself to the top of the second page. When I then try to move some of the shapes and text around it will only allow me to move them sideways, as when I try and move them upwards or downwards it snaps back to its original location on the page.

Is this common when converting from A4 to US Letter? Is there a way to prevent this from messing up the whole document?

Any help much appreciated, thanks.