I understand your concern. Helix3 and Helix Ultimate are different frameworks, and there is no direct upgrade path between them.
Helix Ultimate was developed as a newer framework with a different structure, so migration usually requires some manual work rather than a simple update.Regarding Helix3 templates still being available, I think the main point is that many existing websites are still running on Helix3, so keeping documentation and access available helps users maintain or migrate their existing sites.
It does not necessarily mean that Helix3 is the recommended choice for a new project.For new Joomla projects, using a Helix Ultimate-based template is generally the better approach.
For existing Helix3 websites, the safest method is to create a backup/staging copy and plan the migration carefully instead of making changes directly on the live site.Migration between frameworks is always challenging because customizations, CSS, overrides, and template settings may need adjustments. A gradual migration path is usually less risky than a complete replacement in one step.