You're in the audio section.
And your info about no upgrades without a tear down isn't remotely accurate according to a number of installers (in this forum) and a few pro's locally I've talked to. You can upgrade bits and pieces without going full tear down. Full replacement is by far the way to go, and your price tag of starting at 3k is pretty accurate. I'm not an installer by any means, but I've been digging through this upgrade for a few months now and have gotten a metric ton of options that weren't full builds.
It is your money. Who am I to stop you from burning it for minuscule upgrades.
Yes, you "can" just replace a set of speakers. But they are not the problem in the B&O set up. The cheap 8 watts of power per channel, and shady B&O tuning to try and cover for 8 watts of power is why this sound system sound like crap at any listening level above moderate.
You can add what ever $500 focal speakers you want.....they are not going to fix the problem until you remove the B&O amp, which, requires replacing everything but the rear door speakers.