What did God do to the Lamanites who joined the Nephites in 3 Nephi 2?

He turned their skin white (like unto the Nephites).

(And their sons and daughters became exceedingly fair.)

It came to pass that those Lamanites who had united with the Nephites were numbered among the Nephites; And their curse was taken from them, and their skin became white like unto the Nephites; And their young men and their daughters became exceedingly fair. 3 Nephi 2:14-16