Two actresses played Phoebe - Dervia Kirwan in the first three episodes. In 1997 - 99, Elizabeth Carling took over the role. It doesn't really give an answer on why they made the change.
Emma Amos (Yvonne) - Replacing Michelle Holmes mid-way through the show's 6 series run, Emma took on the role of time-travelling bigamist Gary Sparrow's 1990s wife, Yvonne. It was in the final three series that the character developed considerably, Emma's performance focusing on Yvonne's increasing success as a high-powered business woman. The show's creators also made the character a far sexier proposition than previously. She was often dressed in smart business suits with short skirts or seen exercising in tight-fitting lycra.
Michelle Holmes played Yvonna Sparrow in the first three series, but I've not found why these actresses bowed out and replacements brought in. Sorry I couldn't be more helpful!
