It can be fixed by readjusting priorities of old and new walls. Old items should have a lower priority than newer ones so that new items go over the top of old ones. Turning an old item layer off does not also remove its interference with newer items. I would have thought that a user indicates that he doesn't want to see it when he turns the layer off but only Graphisoft programmers understand why they make such heavy weather of simple, everyday situations.

The way priorities are handled, and the default settings, are a long standing gripe for many users. Join the club!