If your upgrade has cleared, then it isn't refundable once payment has been taken. Only circumstances that I know of where they will potentially issue a refund, is on a fully refundable fare that has been upgraded.

You are bidding to upgrade to a class of cabin, not a specific type of plane. Late equipment swaps can happen unfortunately, so it is definitely something to factor in when deciding whether to bid for an upgrade in advance, rather than trying on the day at the airport when the equipment is confirmed.