Hi N.,
You are right. Too much change could destablize her secure little world. She might not want to go to sleep if she is left worrying about what happened to her routine. All children are different in that aspect. Some will react greatly to the smallest change and others may not even notice it.
If she is very observant, talk with her. Tell her that such a big girl now gets a drink of water before bedtime and that you and Daddy are so proud of your big girl. Give her the sippy cup of water in whatever fashion you normally give her the bottle before bed. If you normally hold her while she drinks, hold her. That way, only what she's drinking and how she's drinking it are changed. You may have to give her milk at bedtime instead of water at first.
If you let her know that it is because she is a big girl and this makes the two of you very happy with her, she may just love the small change and be proud of it. As I said, every child is different.
Good luck.
D.