Few Data Points

Out of Singapore, I have rarely had any problem doing this at check in and I have not been asked to pay any fee or fare difference and this was irrespective of the booking class (in Economy). It used to be standard in out station too but I have observed a recent enforcement of the ticket conditions.

In Tokyo, I was asked to pay 400$ to get on an earlier flight and I declined (about 3 months back)

In Hong Kong a flight back to SG, in PE, the agent told me that they would need to charge me 50 USD/ticket. I agreed to proceed but the was told that this was going to waved "just this time".

I am Gold and not PPS. My personal experience is that in Singapore SQ is quite flexible but again, other pax might have had different experiences.
My feel is that you should be ok.