You can change a ROM only on a rooted phone. As for the second question there are some videos and articles online that show how to unroot your phone but i have not tried it myself to know for sure..
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Re: Oneplus one
I am running Exodus 5.1.1 rom for 2 weeks now.. It is by far the smoothest and best running rom i have tried with good battery life and almost everyday nightly updates! I definitely recommend it!
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Re: Should i load hydrogen OS to my oneplus one
No reason to do it.. At least for now.. I think there are plenty of ROM options like Exodus which are running perfectly smooth for me!
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