running new wire in exsiting conduit

So I have a very old house, almost hundred. walls are ship lap wood, so Im trying not to cut into them if I can avoid it.
Electrical, is cloth covered two wire 1 white 1 black in metal conduit.

question is, is it safe and or legal to pull those out and fish thru romex 12-2 thru existing conduit. or does it have to be mounted differently. all interior electrical. nothing in basement or crawl space.