Well, not sure if you are considering cash ticket or award, but do make sure that your ticket allows changes to the return segment AFTER you have flown the initial (outbound) flight.

From your description, I think that is what you are planning, but that quirk is often not permitted unless you have super flexible fares. I wouldn't count on this being covered as part of the free Covid related change policy.

For awards, unless there is a schedule change or so caused by SQ, changing the return after flying the first flight of the ticket is not really permitted IIRC, though they allow it for PPS / Solitaire members.