Many transgender people choose to pursue hormone replacement therapy, which can cause significant changes in the voice. While these changes impact all people who undergo HRT, they have special significance for singers.
Female to Male (testosterone)
Singer/Composer CN Lester has been doing research on the effects of HRT. They write about this in their blog, A Gentleman and a Scholar. Here is one post about this: https://cnlester.wordpress.com/2013/01/15/singing-and-t-some-tentative-thoughts/.
Singer Joshua Riverdale wrote a great article called Testosterone and the Trans Male Singing Voice. Though dated 2009, it contains great illustrations, links to videos of trans male singers and recommendations for trans male singers that are still relevant.
Male to Female (estrogen)
The impacts of HRT on a male transitioning to female through HRT is less dramatic than for a female transitioning to male. In general, the voice itself does not change range. We have not yet found resources for the pedagogy of working with singers undergoing this treatment. This page will be updated as we gather information.