[From Tatame's page in Portuguese: http://www.tatame.com.br/2011/10/28/Charles-desce-para-65kg-de-olho-em-2012]
According to Ericson Cardozo, one of his coaches, Charles Oliveira is dropping to 145lb for his next fight in the UFC. New training focused on the new weight class will start as soon as November.
After being the smaller men in virtually all of his fights under the UFC banner, his team said he had two options: either drop to featherweight, or change his training to put on more muscle. They chose the former, saying he would not want to sacrifice his speed and explosiveness.
Charles went 0-2-0-1N/C in his last three, so I guess some change was really required, but I do not know if changing his training to put some weight would really be so detrimental to his speed. I hope his team knows it better than I do and he has a good run in his new weight class.