I’m not sure moving this to the SAA forum makes a ton of sense, as it is doubtlessly UA who will have to give you the refund, as they’re the ones who have your money.

However, UA gets to compute the difference between what you paid for and what you actually received, and because of the way airfare is calculated, there’s a good chance that the ticket would have been more expensive as economy + business than business + business, so figuring that value out can be tricky.

In general, though, you were downgraded for less than 5% of the roundtrip distance flown, and still got business class perks (luggage, lounge, etc.). Depending upon what you paid, it looks like business class runs around $6K on that route and economy is around $1500-$2K, so you might eke out a $200-$250 refund; maybe less.