You know, rather than assume you could just ask how it’s possible.

There are times you rebuild something from scratch and old versions of code in that case don’t make sense. In some cases you can copy it over but in many cases it just wouldn’t make sense with the new structure of the project. You basically are rebuilding your product from scratch and prioritizing the most important features to recreate first. You then have to make it worth it for the customer so you build new features too. On the backlog you keep a list of things you likely want to bring back but are less pressing.