We had a leak in the master bath (upstairs) and so now in the midst of
a complete bathroom remodel. Electrical question....

The old bathroom had two circuits flowing into it - the first (15A,
14/2 wire) connected to one outlet (not GFI) and then hopped to
another bathroom where it connected to an outlet there. The other
circuit was sort of normal lighting/fixtures/15A/14-2 wiring, etc.

My question is with the outlets. I want to A) make sure they're GFI,
B) safe as they need to be, and C) add another. With the remodel &
current code do I have to change the whole circuit to 20A? Which
means I'd have to pull all the existing 14/2 cabling and replace with
12/2?

If I do this, then what is the likelihood of me being able to
disconnect the cable at the breaker box, and then pulling the cable up
two floors to the attic through the wall? Is it likely stapled inside
the wall? I don't want to damage the 4 or 5 other cables running down
the same hole through top/bottom plates.

Is a better bet for me to just disconnect the 15A circuit breaker &
tie off that cable so it's dead. Then add a fresh 20A circuit
breaker, and go in the crawl space (very accessible), run the cable
down, then up the HVAC chase to the attic? I think the run length
would actually be about the same.