I've asked a similar question in the past, but I am still a but unsure about the release schedule.

In the past, the upgrade tool on upgrade.odoo.com was released several months after a new major release. This meant that it was possible to start a new Odoo DB with the latest release, but the upgrade was only possible a few months later.

If I remember correctly, this was a bit different with Odoo 18 - the update script was released shortly after the new major version.

Do you know when the upgrade script for Odoo 19 will be available - at least for testing purposes?