The majority of the classes do not need to be altered. However, a few do need some clarifications. Also, the original setting precluded certain races from being specific classes (paladins can only be Anuirean or Khinasi; elves can not be clerics, etc). 5E makes an effort to allow for all race and class combinations. If a DM allows for this, it can be rationalized that such characters are outside the norm for their culture but still allow them; the role-playing aspects springing from such a difference could be interesting as well.
The new classes and archtypes should work just fine in a Birthright game.
Sorcerers, Warlocks and Wizards can all use realm magic. For those other classes that have access to arcane spells they can use true magic, but not realm magic.
5E is an inclusive game that seeks to allow all the combinations of class, race and ability. It is our recommendation that the bard not be limited to the schools of magic they can cast spells from. In addition, we would also recommend that bards be allowed to be regents. However, check with your DM first.
The Domains of the gods need to be defined. Currently there are only seven Domains, but more can be added as they are added through supplements.
The magician was fairly unique to Birthright at the time, but it is easy to implement within the 5E rules. Magicians are the same as wizards with the following changes:
· You can only cast 1st and 2nd level spells from a school of magic other than Divination or Illusion.
· Each time you gain a wizard level, you can add one additional spell to your spellbook (beyond the two you can normally add). However, this spell must be of the Divination or Illusion school of magic.
· When you reach 2nd level instead of choosing an Arcane Tradition you gain both the Divination and Illusion traditions.
There is no real cultural place for a monk to come from. A case could be made for the Khinasi or a secret order dedicated to hand-to-hand martial skills, but monks are not recommended.
The major thing to consider with a warlock are the Patrons. Patrons, by default, are otherworldly beings. However, the world of Aebrynis is largely cut off from the other planes. There are, however, a couple of possibilities. There is The Cold Rider and Apocalypse; these will be detailed over the next couple of days.