This isn't an exact step by step, but the general things to be done:
You would need to make sure current install is up to date
Create a backup of current install.
Setup new Debian server and run Fusionpbx install scripts
Retrieve database password of new server (or change password to match old install)
Restore database from backup to new Debian install.
Restore any custom recordings/voicemails/etc from backup to new Debian install.