I'd certainly agree with Jim's answer above too.
The problem with trying to use the same image is not so much the massively different resolution (1920x1080 against 720x480) but also the pixel aspect ratio.
BD uses square pixels, and what you design is what you get in the disc - DVD on the other hand uses non-square pixels and this can be a design nightmare if you attempt to scale HIgh Resolutiom images down to SD.
Also the button SPHL are different - BD requires a separate layer for each button state in up to 256 coloiurs whereas DVD gives you the whole lot on one 2-bit indexed image with a total of 3 coloirs.
I would definitely suggest creating separate menus for each project.
