The automatic upgrade is now working.
Here's how is declared the model that enable upgrade from 'syntax' to 'backend'.
Note that I do not use the 'compute' parameter in backend but a new parameter named 'migrate_from'. They work similarly except that "computed" values are considered as default value and shown as "standard" vallues in config-model-edit interface. But a migrated value is not a standard value. It just a copy of data that is shown as "customized" values in config-model-edit interface.