My oldest was on pumped BM until 12 months then she refused whole milk so we did 2%. We didn't switch all one day we did 3/4bm to 1/4 milk for a while, then 1/2 1/2 then just a tad of BM to milk. I wanted to clean out the freezer I had so much breast milk.
My middle daughter was on pumped BM until 10 months when they discovered an intolerance to all things milk, so we tried goats milk with the GI's permission and did that for about 2 years until she out grew the milk intolerance. Now she does skim as does my oldest.
My youngest at 3 months old was found to have milk soy allergy so she was removed from BM and placed on speciality formula, she is also a 28weeker so she was having formula added to bm bottles anyway for growth. She did the formula for 15-16 months for growth then went to lactaid milk but it wasn't enough she still reacted horrible so she is now on Soy or Almond. She outgrew the soy intolerance but has a full blown milk and egg allergy.
LIke the previous pp said I believe that when you switch they drink maybe tops 24 oz a day, (3 8oz cups). They get the nutrients and fill from food food, milk/formula is no longer the most important.
Whole is best but I have yet to get mine to take it, even 2% was a struggle because compared to BM it's a lot thicker.
With my daughter, I put her formula through the day (breastfed when I was home) when I went to work at 12 months. She stayed on that until around 16 months when we switched to whole milk.
I gave my son (exclusively breastfed) whole milk at 9 months, once or twice a week because I didn't want to pump any more. We kept giving it to him occasionally until about 14 months and then ran out and just never bought more. He's 21 months now and he breastfeeds frequently and eats well, I don't see the need for whole milk (he also has water and occasional juice).