Cure and Cure II: Single target heal
Cure III: Multi target heal centering on a selected target
Medica I and Medica II: Multi target heal centering on self

This isn't complicated.

Cure and Cure II stay the same.
Cure III is renamed to Cura
Medica I and Medica II stays the same.

Cura is naturally more powerful than regular Cure, which is reflected in potency.

Fire, Fire III and Fire IV: Single target damage
Fire II: Multi target damage centering around the target

For this we apply the same rules as before.

Fire II becomes Fira. The only thing inconsistent with before is that Fira has less potency than Fire, which is an inconsistency that already exists, so it only fixes 1 out of 2 inconsistencies, which is better than 0.

Thunder III is renamed to Thunder II
Thunder II and Thunder IV are renamed to Thundara and Thundaga

This also remains consistent with -ara -aga being multi target centered around your target, but has a similar potency problem with Fire, which is simply an already existing problem that will remain unfixed.

Blizzard is the complicated one. Blizzard III and IV would be renamed to Blizzard II and III, which remains consistent. Blizzard II is does not center around a target, but centers around yourself. As such I would suggest not renaming this Blizzara. Of course, if this is the only one with such an inconsistency, this is still overall an improvement.