Changing the running code but keeping the data in memory is equivalent to changing the code but keeping the data in the database. Changing data-structure defini

Changing the running code but keeping the data in memory is equivalent to changing the code but keeping the data in the database. Changing data-structure definitions is equivalent to changing the database schema. If you want to change the database schema without losing data, you need to write a migration. If you want to hot-reload code with different data-structure definitions, you'll have to write a migration. A hot-reloading solution for a statically typed programming language will at minimum have to detect whether data structures have changed and either prevent the reload or require that the new code includes a function to migrate the data.