You can create a backup of the database you currently have and work your way up from there. To do this, open GuestAll and look for the Utilities tab that is in the upper right corner, click on Backup and choose a directory you wish to save the database in, after that you can proceed by clicking Start Backup of Files. Make sure that the other software you're about to use to manage the data from the GuestAll database can also read .mdb (Microsoft Access Database) files, since currently this software uses that file extension only for the backup process.
