I agree with MEL-traveller, when I looked at changing the SQ LHR SYD sector to SQ LHR SIN I went through the ‘change flight’ process and the cancellation fee was only CAD$150.
As I mentioned, I am very new to Aeroplan so this might be in the T&Cs.
Interestingly, when I originally made the SYD LHR booking, the points required were 130k points and I was short 23k points so I purchased the extra points during the transaction for CAD$498. The current points required for LHR SIN are 93.5k. When I looked at dropping the SIN SYD sector, the new costing was 74k for LHR SIN which should have left me with 56k points but it seems as though they would take back the extra points I purchased in the form of a refund, less the CAD$150 cancellation fee as the refund total in points was only 30k.