An addition or alternative to weapon customization is adaption.
Adaptation works on any item that can be worn by a player and will convert it to your current power level, which means you may either upgrade items with a low power level or downgrade items with too high a power level so that you may use it.
You can only adapt items by spending Platinum coins.
These are obtained by completing quests, which are marked with crossed swords on the world map.
The cost appears to be dependent on the current power level of the item, rather than the difference or the new power level. Items with a power level over around 50 seem to cost ridiculous amounts of platinum coins, which is most likely a bug.
The adapter NPC is found in the Adventurer District of every city, inside the big tower.
Items which are already adapted, such as shop-bought items or self-adapted items cannot be adapted again. Items that you crafted are also considered already adapted.