Can modern figures from legend be used as both marines and army or are they just army And if I was to make them Marines what mods would I have to make to them

Hey Frank, just a short response since I'm at work now.
Out of the box, the Legends figures represent army figures since they have the ACU uniform, and are wearing the vest/webbing the army use (cannot remember the name now)
To modify them to marine figures you would definitely need to change the vest to represent vest the marine corps use (IOTV I think its called?) and possibly change the cut of the uniform to match the marine one.

HTH
Anirudh