I dont think it will hurt a thing. What is your plan for raising them?
Is there tin ductwork behind the grill?
Cold air returns usually just open into the cavity, He may find a 2x4 above if they cut a stud or just blocking for firestop, he can just remove whatever is there and add blocks back in.
Now the clost the cavity below with some stuff that looks like silver cardboard and cut out the plate and floor for these vents, Yeas ago that card board was metal but they have been doing that way since the fifties that I know about.
1850's ?
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found a pic.
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