If a choir is doing a carol service, they often hate if random people from the congregation decide to sing a descant.
Because although in the UK at least, there is one very common descant that many people know, it is NOT the only one, and others might know different ones - and the choir itself might be planning a different one. Or planning some different harmony in the last verse. Or planning not to have one - especially if there isn't an organ.
We did an unaccompanied carol service one time, and a few people in the congregation decided to belt out the traditional descant in the last verse, not particularly well, and it didn't fit at all.
I like the old words too, but there are definitely different versions and not one absolutely official one, depending on which hymn book or carol book you use. And different versions of the harmony, too, or different voicings of the same harmony. Lots of people seem to just assume that the one common one they learned at school is THE version, when really it's A version, that just happens to have become popular because of various famous carol services...