When you say sleeping upright, I'm picturing you standing upright and sleeping like a horse. I imagine you mean with your back elevated. Not quite sure why your feet are elevated and how you can do that if your back is elevated at the same time. I certainly would ask your surgeon with the purpose of this is because it doesn't seem to have any bearing on the outcome of the surgery at this point
I would suggest asking your surgeon what he/she recommends. Every surgeon has his/her own preference. I ask may patients to sleep on their back for least 4 weeks, and avoid stomach sleeping for 3 months.