I will spare you the lecture, as these are unusual times. You should be able to rebook free of charge, until 31 August, irrespective of original fare conditions, but subject to fare difference if any, and any fees that the agent you booked through may charge. You may also wait for cancellation.

In terms of chance of cancellation, SQ are only operating SQ 317 on LHR-SIN during August, but haven't announced their September schedule. Looking at Expertflyer, it perhaps is telling that on 1 Sep, they are selling a whole range of fares on SQ 317, but only the most expensive fare bucket for the other three flights, which may suggest they get cancelled as well during September or at least on 1 Sep. Out of luck if you are booked on SQ 317. Also no flights to Cebu running during August, so I expect your odds of this sector being cancelled are high too.