Personally, I use the following guidelines for converting classes to 13th Age.
- Does it have a different enough mechanics/feel to warrant a new class? If not, choose an existing class and reflavor.
- When using an existing class, do any of the existing talents, maneuvers, spells, etc. work similar to what you're trying to convert? If so, reflavor instead of convert.
- When using an existing class, keep in theme! For instance, a Fighter is likely to have more triggered effects and is a Strength-based weapon combatant. Encounter powers would likely require either a 16+ on the roll, or 3+ on the escalation die, if not some other trigger (or combination of triggers), and daily powers could be either talents or triggered on a 3+ on the escalation die as well as either a 16+ or an odd/even hit on the attack roll. A Rogue-like class could be ported from 4E to 13th Age with less issue; heck, I've used the Rogue class almost in its entirety to convert the Warlock into 13th Age, utilizing the power of refluffing, changing DEX into CHA (or CON) as the primary stat for attacks, as well as taking at least one spell and converting it to 13th Age almost directly.
- Does a class feature, spell or maneuver already exist (more or less) in a different class in 13th Age? Steal it and refluff it! Or at least just mention "this class uses the ______ talent/spell/maneuver, as found in Page XX of 13th Age/13 Ways/etc."
- For 4E implement-based powers that do not automatically scale, look for powers of the same type with the same theme going on; usually there'd be one version for each tier, resulting in three different powers that function almost the same (but increasing in complexity or capability). Group them into one spell, use the Wizard or Sorcerer spells for inspiration on how reformat and convert them, and feel free to utilize feats for particular customizations on a particular spell
- For 4E weapon-based powers that do not automatically scale, ignore the [W] and instead focus on what each power is supposed to do at a given level. Group powers that, just like the implement-based powers, function almost the same, and utilize feats or natural power progression to customize the power
- 4E weapon-based powers do not need to scale in [W] due to the fact that 13th Age already scales weapon damage on a per-level basis (no need to reinvent the wheel on this one).
- Adventurer tier (13th Age) = 1-4, Heroic tier (4E) = 1-10. Champion tier (13th Age) = 5-7, Paragon tier (4E) = 11-20. Epic tier (13th Age) = 8-10, Epic tier (4E) = 21-30. Feel free to convert player resources using that as a basis.