Price and what changes
Withers joins camp after the Dank Crypt, and he still appears at camp if you reach the goblin courtyard without meeting him there. Talk to him in camp to respec.
Respec costs 100 gold. It returns the character to level 1 and lets you redo class, subclass, ability scores, skills, spells, and feats. Race, gender, origin, appearance, and background stay fixed, including a high elf's chosen cantrip.
Resurrecting a companion costs 200 gold.
Patch 5 has Withers raise companions who died before the final battle so they can appear in the ending cinematics.
The oath block and hirelings
A paladin who has broken an oath cannot respec until the Oathbreaker Knight restores the oath.
Hirelings unlock at character level 3 and cost 100 gold each. Up to three can be active. A full party has to swap someone out. Dismissing one sends them back, and recruiting them again returns a level 1 copy that keeps its equipment. The Outsourcing achievement is "Recruit a hireling."
The classes page confirms Withers is the respec: class and ability scores change, and the character starts again at level 1.
Hirelings have their own page
The price above is the companion rebuild. Hirelings are a separate 100 gold, and they are not infected.
Official site: baldursgate3.game.