I friend of us uses a conservative Siddur, the Amidah is different. They have the first Bracha with the Imahot included. I told her that this is not correct and she replied that I should proof it. What can I tell her? I think that there is also a change in their Mussaf.
What kind of proof? If from the Talmud- in tractate Brachot 40b it states explicitly that anyone that changes the form of a Bracha from the form our sages have form does not fulfil his obligation. If from the early Halachic authorities – both Rambam (Kriat Shema 1; 7) and Shulchan Aruch (OC 113; 9) copy this Talmudic passage as binding Halacha. If from contemporary Halachic authorities – Rabbi Moshe Feinstein (Igrot Moshe OC 4; 91) and many others, hold the conservative movement and publications to be against the Torah.