With my first I tried half and half and it made him really sick, so I just ended up doing the switch quick and he was fine. With my second I didn't even both doing the half half thing, I just done a quick switch too and he was fine too
You can do it either way. I bf for 12 weeks and then went straight to formula. My DR recommends when switching to whole milk at 12 months to go half and half if he won't take it right away so I would assume that would be the same for switching to formula. What i did notice was formula was much heavier on his tummy so he ate less.
I started off with an ounce of formula and the rest breastmilk and did that for a couple of days till my babies bodies got used to the formula. Then I increased it an ounce more and waited till their bodies got used to the increase of formula and decrease of breastmilk. Eventually they were only on formula. It's a lot easier on a baby's body. We had to switch brands of formula a couple of times and this also worked when switching brands. We tried just giving it to them straight, but that process was rougher on everyone.
The NICU did it for us. They basically added it slowly as my supply was decreasing anyways. So they kinda worked around my supply and making sure that there was always some BM in there for the first 2 weeks. When she came home she was on full formula and doing fine with it.