After the latest Database data format changes in Magento 2.2.x version, there is a need to convert existing PHP serialized data to JSON format. The new release provides upgrade scripts that convert Magento serialized data. But how to deal with custom extensions, which also use automatic serialization mechanism provided by Magento framework? Thankfully, Magento took care of that too.
The new Magento 2.2 release replaces the usage of default PHP serialized format to JSON format. The release also upgrades scripts that convert Magento data that is stored in serialized format e.g. Magento\Sales\Setup\SerializedDataConverter, Magento\Sales\Setup\SalesOrderPaymentDataConverter, Magento\Framework\DB\DataConverter\SerializedToJson classes. These major changes may affect the correct functionality of already existing custom modules and related data that is stored in the database.