Transferring Spirit Weapon is done through a quest talking to Tarlach, Berched, and Dorren, and there is 2 ways you can do this:
Method 1~ Spirit Body Transfer Ritual-
-Talk to Tarlach with the keyword
-Prepare 30 elemental removals, 5 Spirit Revival Potions, 1 Spirit Liqueur, 1 Spirit Dream Liqueur, and the new weapon you want to transfer on (Have to be 100% and 80% dura).
-Transferred old weapon will become normal weapon keeping and retain metal tool enhancement. The new weapon will not retain the metal toolkit enhancements of the old weapon.
-Old weapon's stat will be NPC shop version of the weapon.
-You cannot change spirit's name
- Weapon has to be within its own class Bow -> Bow or Sword -> Sword
Method 2~ Spirit Memory Transfer Ritual-
-Talk to Dorren with the keyword
-Prepare 30 elemental removals, 5 Spirit Revival Potions, 1 Spirit Liqueur, 1 Spirit Sealing Liqueur, 2 Spirit Dream Liqueur, and the new weapon you want to transfer on (Have to be 100% and 80% dura), and the Spirit Stone of the new weapon you want to transfer on.
-Old weapon will completely disappear
-New weapon will not retain old metal toolkit enhancement of old weapon.
-You have to rename your ego.
- Weapon has to be outside its own class Bow -> Blunt or Sword -> Bow
*Both methods have chance to lower Spirit Weapon's level unless you used Spirit Restore Liqueur before the ritual.
Tarlach's Method decrease a combined of 10 levels (Random) for the 5 stats and 2 fixed for social (12 total) when not using the restoration potion.
Dorren's Method decrease a combined of 15 levels (Random) for the 5 stats and 3 fixed for social (18 total) when not using the restoration potion.
An example for Tarlach's Method: [-5 str -1 dex -1 luck -1 will -1 dex -1 int] 10 levels + [-2 social] 2 levels = 12.
Spirit Dream Liqueur -> Currently sold by Berched
Spirit Seal Liqueur -> Currently Sold by Dorren
Spirit Restoration Liqueur -> Sold in cash Shop for 9900 NX in KR, probably equivalent or more in NA.
*Misc other things to be aware of: Enchant, Repair Protection Potion Effects will be REMOVED when the weapon is changed into a spirit weapon.
Bascially..., method one is you drugged the spirit to sleep again then move it to a different location (weapon) and method two is you extract the memory of the old memory and brain wash the new spirit weapon with that memory.