I've bult my own PCs for 20+ years, and I can't remeber ever really caring about moving the CPU from one motherboard to another. I shop for them as a matched pair, and assuming they work when I get them, I've alays replace both if problems developed later down the road (because a few years later, when you're on the far side of the failure "bathtub curve", you might as well replace both).
But so far you've done the matching. You could put a cheaper CPU into an expensive motherboard or vice versa. You can pick for any one of say 10 CPUs and match it with any one of say 30 motherboards: 300 combinations. If the CPUs are soldered then there will probably be far fewer combinations available, say 50 in total. Expensive motherboards will be paired with expensive CPUs and cheap motherboards will only be available with cheap CPUs.
Like you I've almost always bought CPUs and motherboards together (primarily because I don't upgrade very often), but this move, if it happens, is clearly going to cut down the options we have.