300 years ago, when the post Glorious Revolution legislation was made, the issue of a Buddhist or ex-Buddhist becoming monarch would not have arisen. The population of the country at that time was pretty much 100% Christian, with a very small number of Jews, and the issue was only about Catholicism and Protestantism.
If Prince Charles announced tomorrow that he wanted to convert to another denomination or religion, which in his case I suspect would be Greek Orthodoxy, I think the vast majority of people would say that it should be his personal choice ... but there'd have to be a load of civil and religious legislation, taking up a lot of Parliament and Synod's time to undo the status quo whereby the monarch is Supreme Governor of the Church of England, so I don't think it's going to happen until/unless the situation actually arises.
Interesting that Prince Philip, also technically Greek Orthodox before his marriage to the Queen, converted, but his cousin Queen Sofia didn't.