When contacted (either by phone or mail or onboard form), I've known United Customer Service to automatically (without specific request of kind of compensation) give anywhere from a $75 to $300 voucher for a non-reclining Business seat. The amount of compensation can be based on a number of factors including the individual agent processing the complaint, class of service, flight duration, your status and the fare paid.

And/or upon more direct and specific request for compensation...a Systemwide upgrade for use on a future United flight seems to be at the top end of "normal" compensation.

A refund of the Y to C difference? I'm sure it's happened if someone really became a pain in the butt and fought hard for it. But I've never personally had that happen or heard of it happening.