I'm such a horrible person to give advice about this sort of thing because I'm very blunt with my son.
My husband and I had him throw out his bottle in the trash. Then we made a fun date out of going shopping for those cool sippy cups with the cool characters.
Try to be super duper excited about her growing up and moving on. Give her a new privelege now that she's grown up. People see this as bribes but I see it as real life. We, as adults, can't go on vacation whenever we want. We have to earn the money and earn the time to be away from work - unless you're your own boss but, you catch my drift. I see it as, the soon my son learns that hard work pays off handsomely the better off he'll be when he goes into the real world. I see 30-year-old guys who still throw tantrums and it looks kind of weird.
You're the parent. What you say goes. You're not stripping the bottle away from her because you're mean. It's because you're concerned for her health and overall well being.
If she feels being a baby is much better, try showing her all the cool priveleges she gets being a big girl. She can't help you take care of the baby if she's a baby herself.