Don't just accept a date that only has Saver availability if the date is inconvenient for you - especially when it's SQ that originally downgauged or cancelled the flight. The CSO can (and should) contact Revenue Management to release seats (provided they're not already occupied by someone else) if required. Obviously they don't offer proactively, because this would most definitely involve more work on their part to escalate and coordinate internally - so their standard script is put the onus on you to take whatever other dates with Saver availability only, or accept the downgrade to J.

And if you do settle for J instead of Suites, then yes, the mileage difference should be refunded. Some previous data points suggest that these refunded miles (although expired) are given a limited 6 months' lease of life.