What does bianniu 变牛 mean?
"...第二舰队变牛了以后调到了第二舰队当先任参谋..."


To me this sentence seems a little bit strange. I guess there is a name of the person before this sentence.

If so, then the sentence means that: After the second fleet becomes stronger, someone was transferred and become the chief of staff there.


  "改名后变牛的10所大学"


Translation: 10 College became elite school after changing their names.


  "为什么很多人说读《毛泽东选集》就变牛了"


Translation: Why many people say that you will become smarter/sharper after reading "Selected Works of Mao Zedong"?

So you can see that in these sentences, “变牛” means becomes stronger. The exact meaning might change according to the specific situation.