changing using pfctl at the command line would be very, very temporary.
There isn't a simple way to do what you want from the CLI.
You could use viconfig to hand-edit the config to make the change and then manually trigger a filter reload, but that's not exactly simple.
Best to use the GUI if you can.
If you're looking to automate the process, it would take quite a bit of custom coding to make a command line utility to edit an existing rule in that way easily.