ARCHWAYS in PLASTER WALLS
Maybe I'm missing something, but what you plan to do is no problem. It's just a matter of not getting crazy for risk of cracking walls if your cutting out sections. Other than that we've always just used hot mud to blend/patch into existing plaster walls/ceilings with no problems. If you have to patch in with drywall, no biggie since once it's blended and painted nobody but you will ever know any difference.
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