Brass Plumbing - Rip out as much as one Can

When remodeling.....when I have the walls open, should I be replacing as much pipe as I can or just only as much as required to get the RO to the locations I need. For example, I'm planning another bathroom remodel . Is old brass better than copper or just the same.....

Old brass CAN BE better, but it can also be crystallized and brittle. You replace whatever is "defective" or worn out. "RO" usually means "reverse osmosis", but I am sure that is not what you are running.