Knight is easily the best of the group and should replace any warriors you have. They are largely a statistical upgrade to warriors but unlike their NES and DS counterparts, they don't have access to white magic. They do have the Cover ability and their defend command will drastically reduce damage that isn't magic. They get better equipment and weapons than Warriors as well who should be having a harder time getting upgrades by this point.

Thief is an upgrade from their awful NES counterpart. The can't defend themselves, but they can open doors that usually require magic keys. Goldar's mansion is the place the class really shines. Thief is the only class where the proficiency mechanics come into play as the items they can steal are directly related to their proficiency. Luckily, most enemies only have basic items to steal but it' nice to always have a healthy supply of potions.

Rangers are a pretty decent class that is different from their DS counterpart. They don't have Barrage, instead they are the class that has access to Lv. 1-3 of White Magic, making them good support units. Bow and Arrow mechanics work the same. The real big difference is that Arrows are more difficult to come by in the NES version, most shops don't sell many of them and they need to be found. To compensate for this, Arrows have a significant damage increase. For example, elemental arrows in the DS version have a base damage of 17, whereas their attack power is 30 in the original.

Scholars are a bit rough in this entry. Scan shows an enemy weakness, while Peep shows their HP. They have no access to magic, but like their DS counterpart, their Intellect is on par with the Magus class making them especially potent with magic casting items. Books are good when exploiting elemental damage but are rare, with the first three having to be found and the last three are unobtainable until the Invincible is unlocked. The biggest issue with the class is their criminally low vitality. Not a huge deal in a basic playthrough, but for those hoping to grind to max levels and hp, gaining a level or two in this class will make that nearly impossible. To give you an idea, the scholars vitality stat doesn't even hit double digits in the NES version until you're Lv. 70. Their vitality maxes out at 15 at Lv. 99 compared to the 35 in the DS version.

I'd switch a warrior for a knight, keep your white mage if you have one or switch them for Red Mage if you want variety, black mage can become a Ranger if you want. Simply buy as many elemental arrows as you can comfortably carry on your person for the Hein fight. Keep your Monk the same until the Hein fight and have them switch to Scholar in case you get a level up and make sure to give them an elemental book. In my playthrough, Hein seemed to have a knack for dodging physical attacks so don't expect the knight to be as useful as exploiting elemental damage. Hein also has access to all three -ra spells and no status magic, so you're going to need a dedicated healer for the fight.