Altering preferred connection

[Colin Helliwell <colin helliwell ln-systems com>]

Is there a way to get NM to alter fully on-the-fly which connection is preferred/default for traffic? 
I see that I can modify the route-metric parameter: if the connection is taken down and up again then I see 
the change in the output of 'route', and indeed this seems to change which connection is preferred. 
But is there a way to poke the change through without the down/up? (ideally via NM, or in any other safe & 
sane way)
Ta