|Kevin Ingle ACG Venture-Lieutenant, Indiana—Lafayette aka Sniggevert|
Woran: What about a headband of alluring charisma? Can I upgrade that from +2 to +4, or must I sell the old one first?
That's a flat numerical upgrade, like armor and weapons. Lesser, x, greater does make sense, but is not necessarily the same style of increase.
Can I Upgrade a Named Magic Item?
Named magic items—including specific armor and specific weapons—are not upgradeable. Non-magic specific armor and specific weapons may be upgraded normally. Magic armor and weapons may be upgraded to named versions if they are the same basic material and shape as, and meet but do not exceed the enhancement bonuses of the named versions. Wondrous items whose names include a +X value (such as bracers of armor, headband of vast intelligence, amulet of might fists, etc.) may also be upgraded following the rules for upgrading magical items on page 19 of the Guide to Pathfinder Society Organized Play.
Upgraded versions of named magic items may appear on Chronicle sheets.
Intelligent magic items are not available for purchase unless they appear on a Chronicle sheet or are provided as part of a class or archetype (such as the black blade magus archetype).
The only thing called out as upgradeable are the specific +X items...