Members see a new design and nothing else. Their profiles, groups, messages and posts are where they were, because the data never moved. The things they will notice are the navigation, the spacing and the colours, so a short pinned post explaining what changed and where things are is usually enough.
If you use the BuddyBoss App, members who rely on it will notice more. The app needs the BuddyBoss Theme, so it does not come with you. If the app is how many of your members arrive, decide what replaces it before you switch, and tell them early.
Invite feedback. Ask members to report anything that looks wrong in a pinned thread for the first week. They will catch things on phones and in rarely used screens that you will not.