The biggest change is the visual redesign, which uses a brighter color palette and slightly more blended sprite designs to approximate how the original games would have looked on a CRT (along with miscellaneous small changes, such as a FF6 party member being redone to look closer to the concept art). They also all have new, optional soundtracks. Most notable is FF3, which is a full update of the Famicom version and doesn’t have any similar releases. The old DS 3D remake had different characters and a light story too, so the FF3 Pixel Remaster also has a completely new script for Western audiences.
I’ll emphasize what was brought up already–none of these have any content added in the various ports over the years. Extra dungeons, job classes, FF4’s late-game party change, all that isn’t here.