I mean, take USA for example, is it even the same country after each amendment? After a Civil War? Civil Rights Movement? Landmark Court Rullings?
One could argue that Modern America began in 1965, with the passing of the Civil Rights Act. Or in 2001 when the Patriot Act was passed. Or in 2009 with Citizens United Court Ruling. Or in January 2017 when… you know. Or in January 2025.
When did the country really start?
The Reepublic of San Marino has been independent for a very long time, possibly since 301 ce, that would make it a 1700 years old country that never changed its form of government.