You should save order number in message meta, that way is recommended.
If you want message id same as previous message id, you can update using https://github.com/shamim2883/front-end-pm version. Uninstall current version and download and install github version.
remember if your database copy is already updated you cannot use that database copy. Use database copy before update.
After update finish uninstall github version and install your latest PRO version of the plugin.
Let me know.