Good Q.
For me, the current internal (infernal) debate is whether and why to change the voice. In favor of changing it: 1) I don't actually like my male voice, not for dysphoric reasons, but because it's poorly enunciated and "flat", 2) a change will be absolutely necessary if I want to pass (and I do, at least in general). The reason not to, aside from the effort, is that it sometimes it feels like it'd be a falsehood. It sort of feels like if I have to put in effort to change this, am I sure I'm not role playing when the whole point is to be honest about myself? I think, in the end, I will change it. I'm beginning to practice here and there, especially in the car. The idea that my voice as it happens to be now is "me" doesn't strike me as actually true. For example, I wouldn't think twice about trying to learn to enunciate better.
I imagine that you could practice both voices and maintain the muscles and habits to go back to your old voice. You'll certainly always have the potential to re-learn it since your vocal chords won't change. Personally, I would rather just learn the new one and forget the old one (except maybe as an occasional party trick). I feel like it'll be easier to stay in the new voice and have it grow to be "me" if I don't switch back and forth.
