Option 1: Update your scripts and buttons to refer to the new layout. With FileMaker Advanced and a DDR (Database Design Report) , this can be done pretty effectively. If yo use BaseElements or a similar product to manage your DDR, it's even simpler.
Option 2: After making a back up copy of your file, delete all objects from your original layout. Copy and paste all objects from your new layout to the old. As long as both layouts specify the same exact name in Layout Setup | Show Records From, this should be fairly straight forward. You can select all objects on the new layout and group them to get the top and left position of the group in points on the Inspector's position tab. After pasting, you can edit the top and left values of the pasted group to exactly reproduce the position of the layout objects on the new layout.
Great thanks was hoping there was a safer way but option 2 seemed to work fine. Grouping everything together was a good tip to help maintain the same positioning. Thanks!
It's quite safe if you follow my directions where step 1 is to back up your file. (In FileMaker 13 and 14, you may also be able to undo, but the back up is safer.)