They don't stand for hours and hours they change over hourly between the 10 am start and 4 pm finish . Not every day either as the two regiments alternate duties every 2 days .

Im American so idk but in USA i think the changing of the guards is when one soldier relieves another soldier of his duty while on guard/watch. Im not 100% sure tho since i have never been in the military

Changing of the Guards is when the two regiments of The Household Cavalry Lifeguards in red uniforms Blues and Royals change duty at Horse Guards which they do every 2 days , the regiments line up on Horse Guards Parade at the back and those being relieved of duty return to Hyde Park Barracks . Once on duty the mounted Troopers change hourly the two on foot every 2 hours between the 10 am start and 4 pm finish .