If you’re only interested in upgrading the rear surrounds, maybe you can do a twofer and move the sides to the back, and upgrade the sides. The rears are far less important, but to recommend AGAINST matching is fukkin dumm. Sure, the LCR are most important but I agree with
@mazersteven.
I’ve had Frankenstein systems and sure, they make noise, but having matched, or at least speakers from the same line makes for a much more wholistic experience. It’s not “the front of the room” and “the back of the room”. It’s a SYSTEM!!!!’
FWIW, I’d throw all the Blose in the garbage and replace all 4 if you could. Otherwise, move the sides back, and replace the sides. There are plenty of wall mount options.