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It isn't explained on TV, and it's hardly the only change: He also got quite a bit younger, with black hair, a narrower face, and differently-shaped eyebrows.

The novelisation makes it clear that he wasn't just taking over Tremas's body, he was actually merging his body (which may have looked like the decrepit Geoffrey Beevers version, but still had the 'biodata' of the Roger Delgado version, since they were the same incarnation). One of the later novels explains that the Master deliberately exerted control over the body to make it more like the form he was comfortable with. Both seem like reasonable explanations.