Removing connections from bulkhead!

I'm in the process of replacing and removing some of the wires running to my bulkhead connector. How do you get the wired connections out of the black plastic connectors that clip onto the firewall bulkhead? Is it easier to just cut the wires slide on new male connections crimp and slide into new plastic connectors. I'm having a hell of a time getting the wires out of the black plastic connectors. Can I buy new male copper connectors at any hardware store?

Carl