I was watching the Crown and it occurred to me whether historians would refer to the monarch we call 'Elizabeth I' as simply 'Queen Elizabeth' like 'Queen Victoria' and 'Queen Anne' before the ascension of Elizabeth II. Would Elizabeth I have only been used to differentiate between the new queen?
Queen Elizabeth I was just, as you say, Queen Elizabeth. The new citations and additions of "I" were added after the ascension of Elizabeth II. As an example, if you look at the newer editions of Sir J. E. Neale's classic biography of Elizabeth I, you will see that from the first edition (published in 1934) he mentions that the title of the book changed after the new Elizabeth took the throne. The book title now reads Queen Elizabeth I, instead of just Queen Elizabeth. Books written in the nineteenth century about Elizabeth I or editions of the State Papers from her era are simply referred to as "the reign of Queen Elizabeth".