At the moment, using the bulk change feature to move issues to next-gen project. When a similar custom field with the same name exist on the Issue type custom field there's a prompt for data update. Likewise, if the field doesn't exist, there will be no prompt for data update.
In the above scenarios the data on those fields are removed and needs to be added manually. If the fields exist (In the next-gen project), the data is still removed and still needs to be entered manually. However these fields data are actually hidden, by using the search button view all issues and select the columns to add the field name, the data is still there but if altered prior to the move, then the data changes.
Add a feature that will automatically detect that those fields do not exist and prompt the user for a decision. If the fields that match exist, automatically match the fields to the next-gen project retaining the data.
As the next-gen project provides a Roadmap functionality, which tells a much larger part in Agile Project. it would be better if projects can be migrated successfully than going through the daunting task to re-creating projects from scratch and custom fields which is inefficient.
To manually create a similar custom fields following the available custom fields in next-gen project that mimics the same fields / have the same name on classic project and manually update the data all over again on the next-gen project or during the move. [last resort]