Yeah, GM is pretty savvy about unofficially supporting "outdated" functions like make_color(r,g,b) which is documented in the manual as needing to be make_color_rgb(r,g,b) so you're likely to only need to make minor edits to your game (most notable being that GM8 uses PNG's with native alpha rather than a "transparent color" so you may need to edit all of your sprites to conform to this new feature. Not 100% sure tho)