The change was probably made during the large refit of 1814 -1816, when the beakhead bulkhead was replaced by Sepping's built up round bows design.
It is this aspect that would present the modeller with the greatest problem in converting a 'Trafalgar ' state model to a 'Victorian' model of Victory.
It is not a conversion that I would personally want to make, as the slab sided Victorian Victory is in my opinion her least attractive guise, with increasingly indignities levied on her as the Victorian age progressed. A hotchpot of wooden buildings on her decks, various holes cut into her sides, her interior gutted, fitted with hand me down rigging, and ultimately her masts replaced with those of the old ironclad Shah.
Arthur Bugler summed it up, by reference to her suffering the indignities of old age in the last 50 years before her restoration began.
As a technical exercise however, it would be quite a kit bash.