It's difficult to patch in high build invisibly as it shrinks back when it dries, the edges need to be feathered using a brush dipped in thinners. There's also a very strong chance the colours won't match so the whole elevation will need repainting. With regards to going over what's already been done that depends on the pattern that's on the wall, if it's a heavy detailed pattern you'll probably get away with it but if it's a long pulled out pattern probably not.