Migrate code from Magento 1 to Magento 2
Custom code used by your Magento 1 store is compatible with Magento 2 in most cases. The code will require either rewriting or migrating.
Magento offers code migration tools that can help with this process. Note that due to platform differences, the migration output requires additional work to make sure it can be successfully installed in a Magento 2 store.
Learn more about the Code Migration Toolkit.