Transfering rules for Apple Mail on Mac OSX (without erasing rules already in place)

This question has already been asked and answered, eg.:

"I have a new M1 mini then I’m using OS X Monterey. I have an IMAP mail account. I want to transfer my rules from my old 2009 Mac Pro. If it’s still possible, how do I go about doing it in Monterey?"

"Open Finder, then option Go and then library/mail/V9 (or V folder)/maildata and copy the messagesrules.plist and paste into new Mac at the same location. I believe this will copy all your rules over."

— and here: Moving Apple Mail Rules from one account … - Apple Community

My question is: will this process add to those rules I already have in place on second machine, or simply replace them?

Before I risk it, anyone know whether it will work safely, or nuke my current setup?