J.S.
We were just there - we mixed them for about a week. 75/25 BM/CM then 50/50 then 25/75 then 100%
The mixing we did in the bottle and warmed so that he only had one change to deal with at a time.
After cow's milk in the bottle was established, we did water in a sippy for a week or so.
And then....we ditched the bottles and the heating and did 100% cow's milk in the sippy.
Here's the thing - we began as we meant to go. Meaning that if he pushed away the sippy or the mix - that was ok, but we were not going backwards and offering him 100% BM. He could have water as a substitute, or wait a while until he was hungry enough not to be so picky.
Consistency is key.