You're not possibly referring to an in-flight re-clearance are you?
I know of a couple of times coming across the Atlantic an operator had filed to Montreal or Boston with Toronto or New York as their alternate. When airborne and crossing the coast of Newfoundland they would state they were proceeding to their alternate. Their original destination was always Toronto or New York, but it saved them the fuel as they were going to pass those cities anyways. I'm sure this still happens as it does save a lot of fuel.
The other possibility is you're referring to the "No alternate IFR" allowance, although I'm not sure of the specifics in the US. Here in Canada you can do it when approved by Transport Canada, the weather is VFR for a couple hours either side of your planned ETA and certain weather conditions, even if VFR, don't preclude its use.