Tube naming conventions aren’t consistent enough for you to merely swap things that have a letter difference in for one another.
For starters, the 6CA7 uses an octal socket and the 6CM7 uses a miniature 9 pin socket. That’s the least consequential of the differences between them, so even if you swapped the sockets or got an adapter there’s a bunch of electrical differences between the two that make them non-interchangeable.