Thanks everyone for the comments! This story has a happy ending.

I was able to cancel the original flight online and rebook to an A380. The miles didn’t credit to my account immediately, and it took two phone calls and ~48 hours to get them credited. They didn’t waive the redeposit fee, but somehow I was only charged $50 instead of $100 for two people.

I immediately rebooked on SQ321 (LHR-SIN) which has good availability in F in Feb (new tickets are on the same day as the original flight).

I also found a super cheap repositioning flight to LHR on VS4 for 29k + $260 per person in J. Overall no disruption to the rest of the trip since 80% of the itinerary is after landing in Singapore.

Everything worked out!

Finally, I saw decent availability for F Saver seats (tho generally only one released per flight) on 777 from LHR to SIN in Feb, so I think that’s a much better deal for a regular F seat since there’s no Saver availability for F on A380.