I've always opted for a fourth option to preserve my companion passes when my plans change: change the flight to one far in the future and then change it again once my plans are firm.
In any case, looking at the
terms and conditions of Premium Class paid upgrades, it would appear that they're non-refundable:
If a customer voluntarily changes his/her flight once purchased, a refund for the Premium Class paid upgrade purchase will only be provided if a refundable fare was purchased, in which case the refund will be applied to the original form of payment.