| Mankano Valara |
First off, the Sunbolt ability specifically calls out that it does fire, but also that Crystals can allow it to change to a damage type if the crystal can change the Solar Weapon's damage type. And the fact that the only crystals in the core rulebook merely add fire damage or physical damage, not change damage types, points to it being possible to add fusions to the crystals so that it Sunbolt can be changed typewise.
For the "Corrosive" and other similar weapons its specifies that if the weapon it applies to already deals 2 types of damage, you choose to replace one of the two types with the type the property gives when you activate it.
Going off the assumption that I can add fusion to a weapon crystal,
this means if I have 3d6 Slashing from the Weapon and +2d6 Fire from the crystal, I would be able to make it 3d6 Acid and 2d6 Fire, by activating corrosive. Also, due to how you can choose each time, you could then deactivate it and re-activate it to deal 3d6 Slashing and 2d6 Acid.
Sorry if it seems jumbled thought wise, I am battling sleep deprivation due to college