Red Dwarf: Kryten was played by David Ross in his first appearance, originally written to be a one-off character. When Kryten became a regular crew member, Ross was unavailable, so Robert Llewellyn was given the role. Ross returned later as the voice of the semi-regular "Talkie Toaster".
Gargoyles: Margot Yale was a Recurring Extra in the main series voiced by Marina Sirtis, and became a Big Bad in the third season voiced by Tress MacNeille. Marina Sirtis came back for one episode of that season, but playing Demona rather than Margot.
I don't think these count as examples, because the original Darrin actor never returns to the role of Darrin.
I don't think these count as examples, because the original Darrin actor never returns to the role of Darrin.