Here is one way to handle changing proficiencies. What are your suggestions?
Weapon Proficiency
In the Player's Handbook, you gain all of your weapon proficiencies
and make one full-adventure weapon choice at level 1. This update
replaces weapon proficiencies to make things more interesting.
Damage Group
Find your Damage Group in the list below, based on your weapon proficiencies.
Martial. For proficiency with all martial weapons.
-- Barbarian, Fighter, Paladin, Ranger.
Simple+. For proficiency with all or most simple weapons and a few martial weapons.
-- Bard, Druid, Rogue.
Simple. For proficiency with all simple weapons.
-- Cleric, Monk, Warlock.
Minimal. For proficiency with only a few simple weapons.
-- Sorcerer, Wizard.
Damage Group Level. Your Damage Group Level equals
your combined level for all classes in that damage group and higher. If
your class choices give proficiency in a different group than normal,
that class's levels count toward that group instead.
Race Weapon Proficiencies. If your race gives you
weapon proficiencies, your classes are in that damage group (usually
Simple+) if they would otherwise be lower.
Weapon Type
At 1st level, you gain proficiency with ammunition weapons or either
bludgeoning, piercing, or slashing non-ammunition weapons. You gain an
additional weapon type proficiency whenever your damage die increases
through your damage group. You only gain this benefit if you have not
gained it from the same level or lower in another damage group.
Damage Dice
Subject to your Weapon Type proficiencies and your Damage Group
Level, you are proficient with weapons that use the damage dice shown in
the Weapon Damage table. Once per round when you hit with an attack using one of these weapons, you may add your proficiency bonus to the damage rolled.
You can fight with two 1dX weapons only if you are proficient with 2dX weapons.
Weapon Damage by Damage Group Level
[TABLE="width: 500"]
[TR]
[TD]Martial
[/TD]
[TD]Simple+
[/TD]
[TD]Simple
[/TD]
[TD]Minimal
[/TD]
[/TR]
[TR]
[TD]1st: d4/d6
[/TD]
[TD]1st: d4
[/TD]
[TD]1st: d4
[/TD]
[TD]1st: d4
[/TD]
[/TR]
[TR]
[TD]4th: d8/2d4
[/TD]
[TD]4th: d6
[/TD]
[TD]5th: d6
[/TD]
[TD]7th: d6
[/TD]
[/TR]
[TR]
[TD]9th: d10
[/TD]
[TD]9th: d8/2d4
[/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD]14th: d12/2d6
[/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[/TABLE]
Magic Items, Weapons, and Armor
Magical items can change shape and size to fit any character and can
also trade their magical properties with similar items. During a short
rest, you may exchange magical or non-magical properties between two
items by touching one to the other. For example, a +2 sword touched to a
mundane bow would result in a mundane sword and a +2 bow.
Edit: Added proficiency bonus to damage once per round. Without proficiency, the attacks are weaker than under the PHB as is. Adding proficiency once per round brings it closer to the original power level, and ends up boosting higher levels.
For Fighter class:
Level: New Version vs. PHB = change in damage1st: 1d6 v. 1d12 = 3.5 - 6.5 = -3
4th: 1d8 v. 1d12 = 4.5 - 6.5 = -2
5th: 2d8 v. 2d12 = 9 - 13 = -4
9th: 2d10 v. 2d12 = 11 - 13 = -2
11th: 3d10 v. 3d12 = 16.5 - 19.5 = -3
13th: 3d10 v. 3d12 = 16.5 - 19.5 = -3
14th: 3d12 v. 3d12 = 19.5 - 19.5 = 0
Level: Change in Damage w/o vs. w/ Proficiency
1st: -3 v. -1
4th: -2 v. 0
5th: -4 v. -1
9th: -2 v. +2
11th: -3 v. +1
13th: -3 v. +1
14th: 0 v. +5
17th: 0 v. +6
Edit: Added Magic Items, Weapons, and Armor.