In the FormatSwap Addon Tab, you will be able to find the Material Restore tool. you can use this to restore a selected material to the old version before the conversion process.
But you have to have the FormatSwap backup folder with the FormatSwap Backup file in your blend file directory. this backup file is created when you start a new conversion.
You blend file directory:
Remember that this backup file creation process is automatic if you have “Create a Backup File” Enabled when starting a new conversion
If you have the backup file in a different folder, you can manually place it.
WARNING: If you make a new conversion after another, this backup file will be replaced!