So I'm running 4 Deco M4's currently, but they're starting to show their age a bit. I feel like the main unit is the problem, as my living room where it sits has a lot of devices connecting to it and that's mainly where I would benefit from WiFi 6 and 7. 

So now I'm wondering, would everything work nicely if just bought one BE25 (or BE65 or BE85) for example and replaced only the main Deco with that? Or will the new one basically fall back to whatever standards the old ones support? Of course I'm not expecting the old satelite Deco's to get faster or anything, but if the new main Deco falls back to the standards that the M4's support, there's no point in upgrading just the one. And if I have to change them all, then I might as well change brands as well. (though I have to say the BE25 does look pretty attractive in terms of price)