version 1.7 (and probably former versions)
The upgrade procedure from 1.6 to 1.7 never goes through the upgrade function.
Indeed, the test performed to decide whether it's an upgrade (exists table glpi_plugin_customfields) is never true because this table no longer exists (has been renamed since 1.2).
As a consequence, the upgrade procedure goes through the "installation from scratch" process and recreates all the itemtypes in table glpi_plugin_customfields_itemtypes.
version 1.7 (and probably former versions) The upgrade procedure from 1.6 to 1.7 never goes through the upgrade function. Indeed, the test performed to decide whether it's an upgrade (exists table glpi_plugin_customfields) is never true because this table no longer exists (has been renamed since 1.2). As a consequence, the upgrade procedure goes through the "installation from scratch" process and recreates all the itemtypes in table glpi_plugin_customfields_itemtypes.