There are a few strategies that make this ethical and appropriate.

First, start your own business so that the extra money you are spending for higher fares is your own money and coming out of your pockets rather than your company's or ultimately your client's pockets.

Second, most companies do not mind if you spend your own money on upgrades, so tell your travel group/boss/etc. that you are purchasing the appropriate low priced tickets (and have the receipt), and then call United to upfare. They don't like it when folks try to skirt the rules.

Third, blow through your miles for upgrades.

I had an employee several years ago (before Concur, etc.) that managed to only book Y fares for the extra miles and easier upgrades. I wasn't reviewing his receipts all that closely (who has the time?), but ultimately while on a call with the client, they suggested flying to see us to save $1,000 on airfare. After putting the pieces together, I credited back the client several thousand dollars. He knew what he was doing wasn't exactly right but thought it wouldn't matter since, "we bill it back to the client anyway". I didn't fire him, but he never did it again, and his bonus was substantially less that year than it might have been otherwise.