Kasimir (Kazimieras in Lithuanian) would be Casimir in English and not Charles.
However, many of the time, Lithuanian men used Charles in lieu of Kazimieras in American documents. Men known as William likely had been Vincas/Vincentas in Lithuania. Why they didn't just switch to using Vincent in the U.S. is unknown.
In Lithuanian, the letter J is pronounced as a Y which would explain how Jankauskas became Yankauskas in the U.S.