The change was made after Chamberlain withdrew from the production for personal reasons, Playbill.com reported Friday.
The 20-week tour of the 2005 Tony Award-nominated production kicks off Aug. 22 in St. Paul, Minn.
Bosley is best known as "Happy Days'" Howard Cunningham while Learned played Olivia Walton on the long-running family drama, "The Waltons."
Bosley won a Tony Award for "Fiorello!" and Learned's recent Broadway credits include "The Best Man" and "The Sisters Rosensweig."