What a fantastic job you have been doing keeping up with all that expressing!!
I've been weaning Brenna for AGES (can't bring myself to stop totally), but that's pretty much what I did -one feed at a time. I'm now down to one feed per day. I found that doing it gradually was good in a couple of ways...it was the best thing for my boobs (they would have gotten too sore and I get breast infections really easily) and it also meant Brenna's system got used to formula over time. I know people who have gone 'cold turkey' and they would just express off when they were sore to relieve the discomfort.