If you have 3.0 in your unit, you won’t need to update to 3.5 to install the 4.0 when it is released. You can go from 3.0 to 4.0 but Dan does make a great point, why not install 3.5 in the meantime? It doesn’t mess with waypoints, routes or trails that you have saved. I can’t remember for sure when I updated my units but I think my overlay data remained unaffected. Although, if it does erase it, to reinstall the overlay data is a good exercise in becoming better and faster at it. If you’re like me it’s good for me to “play” with these things often and do and re-do stuff alot so I remember how to do all of it.