No. If you cancel your return flights, having already flown the outbound, nothing immediately happens. It’s up to you at that point. You can ask for a refund, or you can attempt to use the leftover credit for any flight that’s valid in combination with a one-way SFO-HEL fare. It’s only when you do one of those two things that the change fee would be assessed.

However, your refund would not be half the round-trip price, if you were to go in that direction. Your itinerary would first be re-priced on a one-way basis. It’s likely that the one-way price is higher than the round-trip price was, so you’d have zero refund.

If it hasn’t cleared, you can cancel the request. If it has cleared, you might need to pay a redeposit fee, depending upon your status. (I’m not 100% sure if the miles would redeposit for free or not). The co-pay would be returned, although you’d probably have to write in for it.

This is likely to be a far better idea. In most cases, both SFO-HEL and LHR-SFO would price as a half-round-trip in this case. What you’re looking for in the fare rules is whether or not the fare is valid for an open jaw. (Note: you’d actually be looking at SFO-LON fares here, not LON-SFO. You use the fares that are valid for departure from your origin for both legs of a round trip or open jaw).

Note that you may actually be able to do this all on one ticket without taking the scenic route. Some UA fares would allow you to route back from HEL via CPH and LON and add a stopover, so it’s worth pricing out the two options to be sure. Generally, the most effective way to do this is to use matrix.itasoftware.com with a dummy outbound flight. You may need to push the flights back a couple of weeks to account for advance purchase requirements on the dummy outbound; ideally, you’d like that flight to be on the same fare that you actually flew. If you have to do that, just remember the inventory codes on the SK and UA flights, and then look for inventory in those classes.

No, this is an open jaw. If your SFO-HEL fare allowed surface sectors at a fare break point (few do), allowed routing through LON (most do), and had a HIP check* exclusion (many, but not all, UA fares do), you would be able to rebook the return to LHR-SFO without actually repricing to a SFO-LON fare, because HEL-LON would be treated as a surface sector at the fare break point (HEL).

Same as above, although I doubt that you’d be charged a redeposit fee since you’re not actually redepositing anything. Note that UA will charge you either an extra £85 (if they’re still using the old tax rate) or £94 (if they’ve updated their systems) for a return from LHR to the USA in business class or PE as opposed to coach. (If your employer is paying for PE, there’s no additional upcharge to go to business).

*Higher Intermediate Point check. For fares with stopovers — and a surface sector creates a stopover — the Higher Intermediate Point rule requires that the fares be calculated as if travel were to the intermediate point, and, if that would be more expensive, the fare is increased to match. So: if the SFO-LON fare were more expensive, on a half-round-trip basis, than SFO-HEL, your ticket would get that higher fare. The intent is to keep people from bypassing a more expensive destination by adding on a cheaper destination. Most ex-US UA fares actually waive this check; it’s common on mileage-based fares, but most ex-US UA fares have a routing table instead. Since UA can then pick and choose what cities are allowed for stopovers, they don’t seem to have the same concern about people bypassing their fare scheme.

Sorry, yes, I should have been more clear in my answer. If the ticket is non-refundable, you wouldn’t get an actual refund, but you could get an ETC for the difference if you asked them to re-price as a one-way. It’s still the case that the one-way might be more expensive than the entire round trip, though.

If your SFO-HEL fare allowed a surface sector at the fare break point and contained a HIP waiver, then you could use your existing fare, assuming it’s cheaper than SFO-LON. Since it likely doesn’t, you’ll need to refare.

Fair enough. I thought you might be under the assumption that you’d need to transit FRA. I’d say the chances are 99/100 that changing the return to LON-SFO would price exactly the way you’d hope — as two half-round-trips.