Reply by bent

first of all, don’t try it! according to the national electric code, you can only run a maximum of 2 #6 wires in 1” pvc. you’ll need to replace the underground raceway and up size your pvc to at least 1 1/4” to handle 6/3

however, if ever presented with this problem in the future, there’s simple way to do it. unhook the 12/3 on both ends from your panels. you then securely attach the 6/3 to one end of the 12/3, and use the 12/3 to pull the 6/3 through the pvc raceway. if there’s alot of bends in the raceway and it pulls hard, you can use aqua-gel, it’s a lubricant for pulling wire.