Lets take the upgrading weapons like in Aquila through crafting and apply it to other weapons. The Example below will be of the Storm Sword of Kings, but apply your own reasoning to the upgrades of your own School Sword of Kings
Upgrade Sword of Kings (Sharpened) Grants 6 Mega Jolt May Cast Storm Trap 80+ Crit Rating 20+ Crit Block +5% Pierce 90 Level Master Crafter Must be crafted before Sword of Kings (legendary)
Sword of Kings (Legendary) Grants 6 Mega Jolt & Shadow Strike Will Cast Storm Trap after Mega Jolt Will Cast Enfeeble after Shadow Strike 120+ Crit Rating +8% Pierce Level 100 Grandmaster Crafter Must be crafted before True Sword of Kings (Shadow Forged)
True Sword of Kings (Shadow Forged) Grants 10 Reveal Power (Reveal power deals 200 Storm Damage) Will cast King Artorious after Reveal Power May Cast Chain Cast after King Artorious (Chain Cast will repeat cast King Artorious) 200+ Crit Rating +15% Pierce Level 100 Promethean Crafter
This is an example but my point is there. Upgrading the Sword of Kings into A weapon that Rivals the Best Ones in game. Rewarding crafters for, one, holding onto a sword of kings, and two, spending the time and resources to upgrade it to its best form.