Additional Equipment Rules
For ease of play in Pathfinder Society, a masterwork item
can always be upgraded to a +1 item without paying for
the masterwork cost again. In essence, you’re paying the
difference. This applies to upgrading from a +1 item to a
+2 item and so on—you never have to repay the original
cost or sell your current item for half to upgrade to the
next step. Note that this only applies to the same item—
you can’t, for example, turn your masterwork rapier into
a +1 greatsword.
This covers all the weapons, armor, and many miscellaneous items, but leaves a few questions.
Can a Belt of (Str, Dex, or Con) be made into one of Physical Might or Physical Pefection?
Can a Belt of Physical Might be made into one of Physical Perfection?It seems you should be able to upgrade in the first case, but the Physical Might says "bonuses are chosen when ... created and cannot be changed". Does that mean the bonus cannot be increased, or you cannot change the physical stats it modifies, or both? Does that mean it also cannot be upgraded to apply to all three, or it can since it will be become one of Perfection?
Same question for the Headbands.
I think it's referring to the stats that it modifies, not the value of the bonus. Like you cannot change +2 Str to +2 Con.
The wording doesn't actually say that going strictly by RAW then no upgrades or changes, but my guess is the RAI is to prevent swapping the stats being boosted. I'd allow upgrading in my game.
A belt of physical might and the subsequent belt of physical perfection follow perfectly the item crafting rules in terms of multiple abilities on a single item. That is a +6 Con costs 36k as does a +6 Dex. Rules to adding abilities on an item call for the additional abilities to be priced at 50% more. So 1.5 x 36k = 18+36 = 54k would be the cost to add +6 Dex to a +6 Con belt. That makes the total item price 54+36 = 90k (BINGO!) exact price of a +6 Belt of physical might. Add a +6 Str to that belt we just made? Going to be another 54k for 90+54=144k.
You can also use the same formula to mix and match bonuses on your items. Say a Dex+6 Con+4 belt. You cannot however upgrade a Belt +2 dex to a Belt +4 dex.
At least thats how it's done outside of organized play. I'm uncertain if theres any additional restrictions placed on adding abilities to an item in Pathfinder Society.