Up to level 6, there are no differences : leveling up is done exactly as written. After that, a PC gain a level for every 10 000 XP (9 000 is how much you need to level up to 7, but 10 000 is a nice, round number.) Here are the key differences :
- No max HP increase. The only way to raise it is by raising the Constitution score.
- No Hit Dice increase.
- Most abilities dealing with HP and Damage that scale with character level stop doing so (Barbarian’s Rage Damage; Cleric’s Destroy Undead; Fighter’s Second Wind; Monk’s Deflect Missile, Wholeness of Body; Rogue’s Sneak Attack; Warlock’s Dark One’s Blessing.) For the purpose of those abilities, character level cannot be above 6th.
- The Bard’s Combat Inspiration does scale, but it did not strike me as unbalancing, so I say let it continue to do so.
- Spellcasting related stats are going to be modified like this :
- Spell slots are gained normally for every class. Same goes for the Warlock Slot level progression.
- Every spell of 3rd level or less is unaffected. Learn and cast them per the rules.
- Spells of a higher level that do not inflict damage are unaffected. Learn and cast them per the rules.
- Other spells are simply deleted. They cannot be learned nor casted.
- There are probably some of those spells that should be deleted, especially ones that boost damage output or protect from damage. For now, let’s just put a pin on that.
- A more perfect (but time-consuming) conversion would certainly modify some of those spells to keep them, so I would keep an open mind if a player want to have access to deleted spells. Adapting them to keep their damage/healing/protection in line with 3rd level ones is very much possible.
- Casting a spell with a higher level spell slot cannot raise its damage above what a 3rd level spell slot would allow. All other variables (range, duration, number of person affected, etc.) are modified normally.
- Every power that duplicates spells (like the Monk’s Casting Elemental Spells feature) must follow the same rules.
Everything else stays the same!
When preparing combat encounters, a GM would need to consider the PCs are never very much above 6th level. Sure, they would have more ressources than real 6th level (so could probably tackle more combats before needing to rest), would succeed a bit more often at what they are proficient in (since the Proficiency Bonus still increases) and, at very high level (around 17th) would also have access to overpowered attacks like the Monk’s Quivering Palm (but those abilities are all limited to once per long rest, so they are not going to come up often).