Thanks for the reply. I'm not going to do this either way. I just wanted to know if it's possible at all, because I haven't seen people discuss it around here.
If you make the wrong purchase choice, Apple give you 14 days to return it no questions asked for a full refund or exchange.
If you’ve waited longer to realise you made the wrong choice, sadly your best bet is going to be sell your 11 and then pay the difference towards the Pro model.