Rebelreenactor,
Making USMC figures into GIs is not impossible, but if you wanted to do an Omaha beach landing, it would be difficult. Utah would be easier. Let me explain. The 1st and 29th Divisions that hit Omaha beach were wearing their standard wool uniforms with M-41 field jackets over them. However, the 4th ID on Utah wore their cotton, herringbone twill uniforms OVER their wools on D-Day because their HBTs had been impregnated against chemical weapons, whereas the 1st and 29th ID's wool uniforms were.
The difference between Army and Marine HBT uniforms is significant in the top, but not really all that bad in the trousers. Army 2nd pattern (the most common type) HBT shirts have two chest pockets, each big enough to hold a K Ration box. Closures were all with metal 13-star buttons. The USMC M-1941 HBT shirt is a 3 pocket deal with one chest pocket on the left side (with the Eagle Globe and Anchor stencil on it) and two lower pockets on the skirt. None of these pockets had closures, they were just open.
The trousers, on the other hand were different. USMC and Army 1st Pattern trousers were very much alike, but the Army went to the 2nd Pattern in 1943 and the 2nd pattern were much more common. The Army trousers have large hip cargo pockets and no others. These close with a single metal 13-star button. The USMC trousers have two slash pockets on the hips and two patch pockets on the seat, one of the rear pockets closes with a button while the other has no closure.
Now, making USMC figures Army for a pacific battle would be simple, you'd really just have to alter the shirts they were wearing to represent the correct pocket arrangement. GI's in the pacific often had camo helmet covers instead of nets. As for the leggings, Marines wore leggings too, but often discarded them and let their trousers hang free for faster drainage when they'd crossed a water obstacle or hit the beach. The boots the Army and Marines wore during that time were nearly identical. Army M-43 combat boots didn't start seeing combat until January 1944 at Anzio.
Hope that helps!