Just going to start my PhD, found new professor with more exciting project, what to do? I am accepted to a PhD program in engineering in the US with 1st year TA-ship as funding. 

I have contacted several professor, and eventually, there is this one assistant professor (Dr. A) who want to accept me in his lab. I provided my research plan to him before he finally wants me in his team. His research is cool and I think I will be okay with that.

(Un)fortunately, the department just recruit a new assistant professor (Dr. B), which research topic is in my top priority. I love his research to the moon and back! 

What do you think I should do if I want to work with Dr. B?

Until now, I have several choices that I probably take:


Stay with Dr. A :/ (I hope, there is another better solution)
Tell Dr. A that I want to move to another lab (I don't know what/how
to write to him, though)
Ask Dr. B as my co-advisor (for now, I believe that I can intertwine
both professors research, although I am not sure how to convince Dr.
A with this idea)


Any suggestion/comment will be helpful :)

Thanks!
Have you talked with Dr. B at all yet? You need to know if Dr. B would even be willing to take you on at all, and if so whether in a partial or full advisor situation, before raising the subject with Dr. A. If he wants to take on some or all of the duty of advising you, you can also ask Dr. B to advise you on the path to take with Dr. A.

There is not really a good way to say "Your subject is only second choice for me" and it could cool your relationship with Dr. A for a long time to come, so you should be sure that it is worth it before taking this step. Once you are ready to do it, and depending on the available choices, you could lay out the options to Dr. A and seek his input (would he agree to a shared project or not? and similar questions). His answers will then inform your final choice.