I usually try to take my outbound and inbound flights in two different months to make the most of Krisworld's movies.

My experience so far (for flights scheduled near the end of the month):
30th September SQ321 - October's movies
30th May SQ319 - June's movies
26th June SQ225 - July's movies
29th June SQ216 - July's movies

The odds are that SQ are most likely to change their movies earlier rather than later, though simong's example have shown that you can never be sure. I think it depends on the plane. Some planes, they change the selection as early as 26th of the month, so it's really down to luck.

FYI, the preceding flight for SQ321, (SQ319, which flew SIN-LHR), would have left SIN on 1st August as well. Departs SIN at 1240, arriving at 1910, then leaving LHR as SQ321 at 2215, reaching SIN at 1800 the next day.