If the new rules had been in force in 1509 Margaret Tudor would have taken the throne instead of Henry VIII. That could have meant the Reformation would never have taken place and Elizabeth I would never have been Queen.
If the practice had been changed as recently as the last century, Britain could have had two Queen Victorias back to back.Princess Victoria, the Princess Royal would have acceded to the throne in 1901 instead of King Edward VII.
When she died just a few months later, her son Kaiser Wilhelm II would have ascended the throne – something which could have prevented the First World War.
The Queen of England now would have been the completely unknown Princess Marie Cecile of Prussia.