Can parties change presidential candidates?

Can a party replace its presidential nominee after the primaries? "From 1831 to 1968 both political parties nominated candidates in conventions," says Elaine Kamarck of the Brookings Institution. "And while it may take a little while for the muscle memory to come back...there is a process, there is a history to that process, and I think both political parties could, in fact nominate a replacement if they needed to."