Talk about hard to believe -- my twins turn 6yo next month! Wow! I would get rid of the formula first, especially since they already drink milk okay. Way cheaper! I would go ahead and give the sippy cup regularly at meals, etc. But I wouldn't worry about getting rid of the bottle if they are reluctant. At this stage, they are still getting a large amount of calories and nutrition from milk. My 12-mo-olds would drink little bits from the sippy cup, but not nearly enough. I was not willing to deprive them of their milk just so I could get rid of the bottle faster. My twins kept the bottle, especially before bed, till about 15-mos. My doctor didn't seem concerned, and we haven't noticed any problems. I remember being a little stressed that I couldn't switch them to the sippy cup faster, but it just happened without trouble once they were ready. What seemed impossible at 12 months was a breeze just 3 months later. Good luck and enjoy!