Just from my own experience I found it better to switch one milk and leave the other alone for a while because otherwise my DS's milk intake would have dropped too much and also he would have screamed his head off in the morning.
BT milk was an easy switch at 12 months, I did small solids supper with cup of cows milk before bath time then over a period of about a week offered less and less for the BT bottle and took a bottle of water up to bed instead in case he needed a sip. His milk intake dropped a lot as he'd take a much smaller amount with the sippy cup. We still had lovely BT cuddles just without the bottle there.
Morning milk was a different matter. He would have thrown a fit without the large amount of milk and easy to drink. I did this one more gradually over a month and the bottle was gone at 13 months. Began by sitting upright on me for bottle, and I opened up the teat so it was more drinking than sucking. Tried many times to switch to a straw (in a bottle/cup) instead of bottle with teat, took several tries before he agreed. We continued with a large morning milk on my knee, sat up, using straw for a long time.
I did not mix formula and cows milk because my DS just took the cows milk, he wasn't bothered at all by the switch.