Just wanted to add my datapoints for anyone interested. I just got my upgrade to Gold today, Feb 19 in the US. I flew my 3rd and 4th segments beginning on Feb 10 in Malaysia, so Feb 9 in the US. So it took about 10 calendar days. Not bad considering the CNY holidays, I guess.

I had ~23,600 Elite miles towards requalifying for Silver after my last flight segment (started from zero after getting Silver from the Marriott promo, but I think I had approx 22000 miles before that). After being upgraded to Gold, I am 11,300 miles towards requalifying for Gold. I have no clue how the math works, because I was expecting to restart from zero, as you had stated. Anyway, I'm about 39,000 miles away from requalifying for Gold, so I'm happy. I'm not likely to fly SQ again after this year because my projects are winding down, but the status through 2/2026 should be useful for other travel.

Additionally, I can confirm that 4 segments on one booking satisfies the challenge. I flew LAX-EWR-SIN-PEN and returned PEN-SIN-LAX. The LAX-EWR segment was on a partner airline, so did not count towards the challenge. All the other segments were on SQ metal.

Anyway, just documenting my experience for posterity before I forget the details. Maybe the info will be useful to someone.