Do you think that making a ruling that fireball ties your shoes is "making a change to an existing rule"?
I would say it is: It's adding a function to the spell it did not previously have. And I would say the same thing for causing it to set fire to worn or carried items. We have a description of how the spell works, and a list of the things it may set on fire. That it doesn't specifically state that it doesn't set other things on fire is no more relevant than that it doesn't specifically state that it doesn't tie your shoes.
I would say it is: It's adding a function to the spell it did not previously have. And I would say the same thing for causing it to set fire to worn or carried items. We have a description of how the spell works, and a list of the things it may set on fire. That it doesn't specifically state that it doesn't set other things on fire is no more relevant than that it doesn't specifically state that it doesn't tie your shoes.