By way of update, my colleague spoke to the supervisor on duty in DXB last night when checking in.

She told him that if he were downgraded to economy, he would be entitled to a business class ticket in line with the IDB rules. She mentioned that he would have to go to a ticket office (a gripe for some on here...) as it is a requirement to present the physical voucher when redeeming.

He said he would accept the compensation in the event of a downgrade and so she noted his preference on his PNR.

When he got to the gate... he discovered he had been upgraded to F (flex business, gold) as apparently there were three last minute no shows.

I guess this leaves things somewhat unclear given conflicting reports. Is it simply a matter of YMMV as to compensation? And if you have an IDB voucher, are you entitled to use it to redeem a ticket within your specific fare class only (so I, in his case) or if there is only one ticket left in J fare bucket, you could use it..?