do it at the airport. go there little in advance and ask at the desk. they'll have to work with the ticket office and send you there as well, but it can work.
thye told me that normally at check-in time they have access to all remaining seats and can easily handle.
i did like this in MXP and was blessed by super nice staff that worked well for me and wife and got the upgrade done in 20 mins by using E+ vouchers.
little patience and a couple of signatures and all was nicely done