I don't want to start a fight................... but I just wanted to say that over-tiling can be a perfectly good way of going about it. Perhaps not exactly the "best" way, since it will depend on just how solid the existing tiles are.
It's one of those contentious issues where nobody is either "right" or "wrong". Merely a matter of personal choice.
Beso......... the glaze actually makes no difference at all with modern adhesives. In fact, on many occasions, I've plastered over existing tiles which rock solid and difficult to remove. With PVA and Carlite "Bonding Plaster", the result is as good as any other regular plastering applied directly to wall surfaces.