I was just about to post a request for exactly the same feature - so a "me too please" vote on this one. It would be so very useful to be able to do this during composition when things can change so much. By the way, if it is implemented I think it should be possible to go ether way ie double the durations or half them.

Ideally if it could be for a selected (highlighted) number of bars up to the entire piece would be excellent.
regards to all.
-johnsm