There is no way to change your characters' classes but getting the arcomage deck is very easy. If you have trouble with Emerald Isle, use the Day of the Gods pedastal on the hill with the dragon's cave, and the fire resistance pedastal right outside it. Get everyone the bow skill and bows, and you should be able to ship out to Harmondale the first day.
Once in Harmondale, just do the stone skin pedastal outside the inn and then go right back to your castle without fighting anything. Go south, up on top of the big hills there, and when you get the road that cuts across it, slowly go down the slope, moving side-ways along it, or use the spell, feather fall, and enter the small village. Go south along the road from here and follow the path that leads off the road right away. Follow this path to the White Cliff Caves, running right past the goblins you see, and inside the cave, just run past everything again as you go straight west and up to the north end of that passage. Then run back, again dodging everything, and go back to the village south of the town around your castle. Climb up the hill in between, using the jump button to get a head-start (and just hold down the forward movement key until you finish).
That's it. I hope this helps.