Confirmed upgrade flow
Recover the Etching Needles, deliver them to Franz, equip the Tarstone you want to train, then earn its experience through combat. Later upgrades require additional resources including Tarcore.
Why a full bar may not be enough
Experience and upgrade access are separate gates. Check that the service is unlocked and that you have the required currency or Tarcore before assuming the interface is broken.
What is not published as fact
Current guides use inconsistent level numbering and maximum-rank language. This page therefore describes the reliable flow but does not state a definitive number of levels.