Regarding that last, there have been at least two cases where a secondary mission was added later, after launch. Pioneer 11 was launched for Jupiter, then reta

Regarding that last, there have been at least two cases where a secondary mission was added later, after launch. Pioneer 11 was launched for Jupiter, then retargeted during flight to slingshot to Saturn too. And New Horizons was launched for Pluto, then retargeted during flight to encounter a particular Kuiper Belt object (which hadn't even been discovered at launch.)