I think you are using a different program than I do. Mine is SMS Backup and Restore by Ritesh Sahu....I think yours is SMS Backup+ by Jan Berkel?
So if you have your old phone still, I'd restore all the texts on it (I am assuming that would still work)...install the program I use...back it up and upload the backup file to dropbox or google drive.
Then on your M8, install the program I use and then bring down that backup file from Google Drive (or Dropbox) and get all of your old texts on the M8
But good idea i still have not sold my G2, so ill try to restore it and use ur app.