How many miles did DL charge you for the SQ ticket? Is it round trip LAX-SIN in F?

You may not ultimately get the SQ F seat that DL "sold" you but you want to be sure that you don't use all those miles for an inferior product. If the best that DL can do is offer you LAX-NRT-SIN in their J -- a huge downgrade from SQ F -- then get a refund on any miles beyond 120K. Even if you get put into SQ J then you should get some miles back. Plus I would think you're due some goodwill compensation from DL for this hassle.

I would NOT wait until day of travel and hope the agents at LAX will sort it out for you. That's a crazy strategy unless you don't have to be in SIN and are OK to just go home if it doesn't work out.