Is there normally a rough kind of time frame? Eg a few days before your flight etc. Do you get an email or do you need to keep checking?
Mmb currently shows this from wtp to business. I think that's just a normal upgrade and obviously I wouldn't be willing to pay that.
I'm able to add 9 rt business tix to my basket for the above flights so I know for now there's definitely lots of availability (unless that's a terrible way of checking). Still got a few months to go