it's not all that sudden; the change has being going on for years. The spelling isn't really relevant, since I don't think her name was ever written down in Brythonic; but the new spelling gets us closer to how it was probably spoken.
and Ceylon is now Sri Lanka - am cheesed off at all these changes and refuse to acknowledge them - so there! Bad spellers think they can change my language - loyk 'el!
I always understood that 'Boadicea' was a Victorian error in translation and the later version is the correct form.
Another example is the word 'pharoah' which the Victorians assumed meant 'king'. In fact the ancient Egyptians used the term 'Pharoah' to represent governmental power in the same way as we use 'the opinion of 10 Downing Street' or 'the opinion of the White House'. 'Pharoah' has since been adapted to mean 'King' - most notably by modern Egyptians as they realised that western tourists preferred the more romantic term rather than just 'king'.