Originally Posted by lokijuh

I assume you are chasing under EU 261 rules? I am not sure the specifics of how it works as it relates to connecting flights, but as the sector was <1500km, that would mean a 30% refund of that sector of the ticket - so 30% refund of 6% of the ticket (if return ticket) means a refund of 1.8% of the overall ticket cost (3.6% if one way ticket). But I could be wrong on my calculations, if it relates to overall distance you could be due 75% of 6% (4.5% of original ticket cost).

If chasing without reference to EU 261, I suspect the cost of a business class flight connecting from CPH-SIN in business to BKK-SIN in economy is probably the same as, or even more than, the cost of business class all the way through, so their systems will probably say no refund is due!

Maybe it would be better to ask for some form of compensation (miles?) or customer service goodwill gesture for the downgrade as it was due to factors outside your control, rather than asking for a "refund".

Having said all that, SQ customer service at the moment is extremely slow. Plenty of threads about that on here.