Before transition I was a classically trained, semi-pro concert tenor. I didn't do operas, but I was a ringer for a few old-school Catholic church choirs on big holidays. I have experienced this loss of the lower register as well; however, I don't think it has anything to do with hormones so much as the standard "use it or lose it" adage in the vocal world.
The secret to a female voice put into singing terms is pretty simple: access the head voice and speak through it. Men speak through their chest voice: a rumbling, raspy, deep part of the voice created by testosterone. When we start speaking in female tones, and stop using that chest voice so often, the power of that portion of the voice slowly fades away. The same happens when you stop singing for a long while - the range of the voice contracts.
The voice doesn't change. I can still access my full tenor range, mind you, minus low G and a clear low A. The difference is where my breaks lie: I have a nasty break around F3/G3, which pretty much makes singing anything remotely baritone impossible; however, my voice is break free from A4 to A#5, and I have a richer, more "usable" falsetto up to about G5, so I can pull off alto in my daily voice and second soprano in falsetto. The timbre of my voice, with training and daily "passable female voice" use, now leans more female than male.
I guess I'm just saying "the hormones didn't do it: your vocal practice did it." The latter is far more impressive than the former, however.
