Would any flight with Iberia flight numbers be an Iberia ticket instead of 125?
The most important thing to remember is that even though the flight may be operated by BA, if you have bought the IB flight number you have bought an IB flight.
If all of the flights being sold on this ticket are IB flights, then I think it's very likely to be an IB-issued ticket.
In any case, if you look at section 03 of the post that you've been directed to, if you're trying to upgrade an IB flight then:-