I have a player who's playing a Mystic from Unearthed Arcana: Psionics & the Mystic, Take 2. The Mystic class has been recently updated to a complete class, and some very significant changes were made in Unearthed Arcana: The Mystic Class.
The character my player is using currently wears heavy armor, wields a greatsword, and has invested feats into using both of those things well. This worked well with the previous version of the Mystic, which allowed for proficiency in heavy armor, and synergized well with the greatsword by giving options that don't rely as heavily on dexterity for survivability as the current version does.
Other players have expressed some level of discomfort with the version of the class we are using now, such as it completely outperforming the paladin in ways that the paladin should be built to shine–by adding significant amounts of damage on hits, similar to the paladin's Divine Smite. Some of those concerns would be mitigated by moving forward with the update, but it also seems to open up a lot more concerns, such as overshadowing the wizard in a similar manner, by taking Mastery of Fire, which allows the Mystic to use Detonation, which outshines the wizard's signature Fireball and control spells by doing both at once for the same cost.
We discussed before the game started that changes might be made to the class, and had agreed to review and analyze those choices before settling on a course of action. However, we had not anticipated that the changes would be so severe as to change the identity of the character.
I see several options forward, but none seems to be a perfect solution to maximize the fun of the effected player, and the balance against other players.
How can I mitigate these identity-altering rules changes while keeping the game fun for the player affected, and well-balanced with the other characters?
Ideally, answers will be backed up by experience with a similar situation, in this system or others, and will provide–or help my group arrive at–a solution to help improve the game experience for everyone affected.