The way users are defined and used differs greatly between LedgerSMB 1.3 and older versions. In version 1.3 user access to the database is enforced by the database itself. This means that users logging in to the LedgerSMB web application are in reality logging into the PostgreSQL database. In older versions, the web app would verify the user’s credentials (using a common database connection used for all users).
The difference between these approaches is that security is no longer (solely) maintained by the web application - with all inherent risks. Instead, the database now plays an important role as well. The effect is that the LedgerSMB team now leverages the experience of the PostgreSQL community - a highly respected community well known for its security focus - to make sure your data stays secure.
This structure also enables LedgerSMB 1.3 to offer separation of duties and authorizations throughout the application without being required to do a full rewrite of the application.
It’s this shift in paradigm that makes it impossible to meaningfully migrate users from older LedgerSMB and SQL-Ledger versions to LedgerSMB 1.3.