No such requirement. Not only can Delta change equipment on the DCA-SLC route if it chooses to do so, it could even start the new service with a different aircraft type. That's what America West did -- after promising DOT in the DCA beyond-perimeter route case that it would start DCA-LAS service with 757s, it actually began the service with A319s. To my knowledge, America West was never penalized by DOT for this bait-and-switch tactic.