Presumably Queen Elizabeth 1st has only been called that title since our present Queen came to the throne, taking the title Queen Elizabeth 2nd (presumably in her lifetime Queen Elizabeth 1st would have been known as Queen Elizabeth). What is the word used to denote this change.
There is no specific word or phrase for this.
You could describe this a the redesignation of Queen Elizabeth. A change made retroactively, or in retrospect. And you could say that the change was made to differentiate and disambiguate between the two Queens with the same name.
Elizabeth I lived and reigned under the name "Elizabeth of England" and that remains the nearest thing there is to an official Royal name. The numeral "I" is just inserted by historians to avoid confusion.