Navigating a route, but not from the beginning

This is more of a Zumo question than Basecamp, but there is no English forum for motorized navigation (why?):

I created a route in Basecamp, using mostly shaping points (do not alert) and a few waypoints. The map is identical to my Zumo, and the routing options and avoidances are set exactly the same. I met a friend about 30 miles into the route. The restaurant we wanted to go to (waypoint) was closed, so when I activated the route, the GPS asked me which was my next destination. I guess it realized I was not anywhere near the starting point but rather along the route already? I chose the next waypoint after the restaurant. The Zumo recalculated and in doing so threw all shaping points up to the waypoint I had selected out the window! Thank goodness I had also uploaded the track, so I could follow it instead, under heavy complaining from the routing software along the way ("make a legal uturn"...). How could I have done this differently?