Pretty much the only thing you'll need to worry about is the chipsets, because the autotiles are separate now, plus RM2k3 separates upper and lower layer tiles.

The charsets are easy enough, just strip the right-most collumn and either combine it with with 7 others or just paste it in the upper-right corner of a 288x256 PNG image.

Those are the only things I can think of right now that would require some work to do..