One thing we don't know is how they would treat Charles becoming King which happens simultaneously, although the Coronation would likely be a year off. You'd imagine they'll be profiles of him alongside those of the Queens. And as for football - we're surely all looking forward to watching the England team and fans remember to say "King" rather than "Queen" when singing their anthem. Suspect it'll be "God Save the Quing" for a while
The Coronation doesn't have to be a year, it can be pretty much anytime - Elizabeth's was 15 months between taking the throne and the coronation. George IV seems to hold the "record" between throne and coronation, just under 18 months.
There are all the other things that would have to change as well, not just the anthem. You'll be carrying Prince Charles's face around in your wallet at some point, put his head on future letters, new insignia will be needed for the police and military, passports, post boxes... None of this will happen overnight (since I believe some older post boxes still have Victoria's crest on them) but it will happen, and because we all got used to seeing the Queen's head on a coin looking to the right, tradition dictates the new King will be looking to the left. That on its own is enough to cause enough of a panic in this country where we fret if we run out of teabags in the office
It likely be - if the Queen dies before September the following summer. If she died in say December it would be 18 months or so.