Bid to upgrade seat option

I've just logged into my booking, for next May to MCO to see if anything had changed ref the new seat rules, and noticed that the option to bid for an upgrade has suddenly appeared.

Strange as the seats are pure reward seats and not paid for - i thought you could only bid on a cash ticket?

Having played with the amounts it says £1000 each has a good chance of being accepted and £1200 a very good chance. Bearing in mind I've already paid circa 80k miles, 2 AMEX upgrade vouchers and the taxes/surcharges, this makes in my view a good deal. The pure money price for the same flights is £5300. So about a £2500 saving

Now will Mrs Chumba let me spend the cash ! I feel the credit card burning a hole in my wallet.

That sounds an expensive way to upgrade (I am assuming you are going from PE to UC). £600 each way it not a bargain.
If there are seats available why not buy some extra miles and upgrade that way ?

Hmm -- that's odd. I thought you could not upgrade a miles ticket with cash (other than perhaps at the airport if you were lucky).