I have a couple questions about the rules to upgrade items in PFS:
1) Can inferior ioun stones be upgraded to normal ioun stones by paying the difference in price?
2) A belt of giant strength +2 can be upgraded to a belt of giant strength +4, but can it be upgraded to a belt of physical might +2? It doesn't change any of the item's prereqs, just adds new ones.
No to both. The upgrading of +X items allows you to only go from +X to a higher +X. Its fine to go from a belt of giant strength +2 to a belt of giant strength +4, but not to the belt of physical might. And for the ioun stones, they're not +X items, they're just unique magical items that provide a +X benefit so they're not upgradeable.
|Andrew Christian Venture-Captain, Minnesota—St. Paul|
Actually Mourne, you are half correct.
You cannot upgrade Ioun Stones from inferior to normal, because they are different materials. The base materials can't be changed.
You can, however, upgrade a belt with a +2 Str to a Belt with +2 Str & +2 Dex or +2 Con and later add the 3rd at +2. Should you want to upgrade to +4, then all stats modified by the belt must include the upgrade. So if you had a +2 Str/+2 Con you'd need to upgrade it to +4/+4. If you wanted to add Dex to your +4 Str/+4 Con, you'd need to add it at +4. You cannot have an item that is +2 in one stat and +4 in another. This is also true for the mental headbands.