Charge for changing fare class?
 

(I tried searching for this, but search isn't working.)

Suppose someone books a domestic UA flight for me. They are paying for it, so I have no control over the fare class. It turns out to be an S or T fare, but I prefer it were something else. Would UA charge me the $100 change fee + difference in fare to change the fare class, or just the difference in fare?

Domestic fares would require a change fee regardless of what you are changing it to. Generally speaking international fares would allow for higher fare without change fee.

Only those of W and above, I think.

If you call immediately after ticketing you may be able to get it done under the 24 hr rule. You can also explain the situation to the agent and see if they will make an exception.

I did this on another airline where a discounted coach ticket was purchased for me, but I wanted to up-fare into F (for a couple hundred extra dollars). I explained the situation to the agent (mentioning that my company will only pay for lowest cost Y, but I wanted to pay the difference). After a brief conversation with a supervisor they charged the difference to my credit card and thanked me for my business.

Haven't seen anything to confirm that. The S fare I'm on allows for no penalty to higher class.

Almost all W and above
few S/T/K/L

allow free upfare.

Usually, promotional STKL fares do not allow while "normal" S/T fares in cheap markets do.

Maybe it's market specific.

T fare NYC-NRT waive for upgrade to higher fare.
S fare (recent European sale) NYC-MUC waive for upgrade to higher fare.

Could be that too.

Like other fare rules, they follow patterns, not absolutes. It's good policy to read the fare rules before you purchase a ticket. I once was about to buy an ORD-PHL for $192 RT and after asking what the rules of the fare were, the agent volunteered a WA fare for $109 each way. The WA fare was 100% refundable and thus the better value for me.