i replaced all 4 because after replacing the front springs alone the car looked preposterous, like it was doing a pushup. i think if you did the opposite the effect would be quite different though. a bunch of years ago i had an 81 240 and the rear springs were tired, and fronts were fine, so i did just the rears and it settled to a nice rake with a slightly hunkered front look to it.
on my current car i gained 3.5 inches at first in the rear once i did it, its settled about half an inch since then. my initial rear ride height was a little lower than the yours all around though

i was forced into it, but it worked out because at the mileage the car was at, the rear springs were heavily worn with flat spots and corrosion.


EDIT-
actually I take that back, in the rear both are cars were about the same initial height in the rear

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