Is it possible to change or destroy goods that a scout is sending?
From the FAQ 2.0, page 2, we have the following:


  Q: Can a scout gather icons and/or resources from a square  in another
  city’s outskirts? What if the square contains a  building or a wonder?
  
  A: A scout can gather resources from a square in another city’s 
  outskirts, even if that square contains a building or wonder. If  the
  scout sends those icons and resources to a different city,  the
  original city cannot also use those icons and resources.





  Q: When a scout moves into or out of a square with a coin  icon, is
  the coin total updated immediately?
  
  A: Yes. This can result in an economic victory.


Given this, it appears that two things are true.  First, when a scout moves, the tile it is sending to the city also updates (as evidenced by the coin example).  Second, if a scout is on a tile of one city, the resources from that tile can be routed to a different city or not, but cannot be double counted.

It would appear from this that the behavior of the scout is to have the tile it is on considered part of a city of your choice.  If the scout moves, the resources it is sending are changed to the scout's new location.  If the tile the scout is on is modified, the resources the scout is sending also update (exactly the same as if that tile were in the city radius of the city the scout is sending resources to).