Can I install ArcaOS on top of an existing eCS or Warp 4 installation, as an upgrade?

This type of installation is known as a migration, and not an upgrade. ArcaOS 5.0.x cannot be installed in this way.

The recommended way of installing ArcaOS to ultimately replace an existing eCS or Warp 4 installation is to install it to a new partition and then reinstall applications and move data as necessary, building an entirely new working environment.

A migration feature is planned for the ArcaOS 5.1 release cycle.

For more information concerning updates, upgrades, and migrations, see this wiki page.