This is why I find G9 superior, as it gives the freedom(s) to do stuff like this. Unfortunately, I recall your hardware not being super high-end, so it'd be tough working that into it with limited VRAM. If it's pure transformation (as opposed to sissification), then you might be able to get away with a different look, but obviously that's said without knowing context of the said story.
The most realistic option is either going with some of the trans options out there for Daz:
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,
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,
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,
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,
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. You could take Jackie/Chrissy (linked), remove their shapes and add another normal male shape to them, and then switch them back to the trans version when the transformation happens. The other option is using the UV Swap (linked), which will allow you to use the male figure and bring the skin texture over to a female body/base. Using a LIE-based tattoo or something of the nature, to signal that it's him in a female body.
It's a little bit heavier texture-wise, but 8.1 has a couple masculine/feminine version of figures.
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,
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with a small breast morph could probably pull it, too. Not sure if there's too many twin sets for anything below G9, but that might be another option.