Replacing house wiring low roof?

You didn't say why you feel the need to do this. As much trouble as it seems to be, re-feeding the receptacles from below it is what I would do. Indeed have done. It requires some thought when drilling up from below to insure you are in the wall and not the floor. Probably necessitates removing the existing boxes and installing new "cut-in" types. But they are cheap and a lot quicker than trying to fish the wire up through a small hole in an existing box. Some wall switches can be reached from the attic crawl space, others from below. And yes, I would run 12/2 romex to everything. The existing cables (you don't have pipe do you?) will be strapped 2 or 3 times above the box and you won't be able to use them to pull new wire in. I'm going to suggest looking for some help here. Perhaps a neighbor, friend or co-worker can suggest someone you can hire to help. Unless you do drywall work for a living you will find that you can't match the wall texture perfectly. Having tell tale signs of patch work all over the place is not what you want in the long run.

Good luck with it.

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Replacing house wiring low roof?