TIL you can mix/match speaker impedance

I was a bit surprised by the answer because I’m pretty sure I’ve seen the opposite guidance elsewhere, or I was missing something in the nuance before. Definitely news to me, so well worth passing on. Enjoy, curtesy of AVS Forum.

Q: I have a 5.1.4 speaker system hooked up to a Marantz SR7010 AVR. My speakers consist of Yamaha NS-777 mains, NS-555 surrounds, NS-444 center, and NS-333s for the elevation channels. I want to upgrade the mains and the center speaker. Since all of the speakers in my system are rated at 6 ohms impedance, does that mean I have to upgrade to other 6 ohm speakers, or can I mix impedances?