the 'D' class automatically becomes First-class (with 150%) on UA's two-class-only inner-USA-Canada-Puerto Rico routings - it is downgraded to eco (with 100% only) on Mexico-USA and on mainland-Hawaii segments.

And outside N-America you CAN't upgrade business 'D' to international first (if they apply the official ruels, sometimes you can at check-in - against the rules).