It would seem that weapons accept spirits within a certain power range of the weapons power. The power of the spirit has to be at max equal the power of the weapon. There also seems to be a limit for how low power the spirit can be. Anyone else know anything about this? The highest unfittable spirit cube i have below my power 66 weapon is 52.
I found a way to remove upgrades from a weapon, just pull the block/spirit away from the weapon and click outside the window (like if you throw something out of your Inventory). The upgrade is gone and you got a free slot back, but the block/spirit is completely deleted.