Can an Arcane Bonded Item be upgraded?
A character with the arcane bond class feature may create a bond with any item he owns, either magical or mundane, as long as the item falls within the categories permitted by the arcane bond ability (the cost for bonding with a new item still applies). If a caster later wishes to upgrade an existing bonded item, he may do so for the cost (not price) of the final item as listed in the item's statblock.
For items which can be enhanced incrementally (such as weapons or a ring of protection), the caster must meet all prerequisites for the item as outlined in the item crafting rules. For example, a nonmagical bonded dagger can be enchanted to a +1 dagger for 1,000 gp instead of the normal 2,000 gp, but the caster must be at least 5th level (a prerequisite for the Craft Magic Arms and Armor feat). To upgrade the item further to a +2 dagger, the caster must have a caster level of 6 or higher (three times the item's enhancement bonus).
A bonded item that is enhanced must still conform to all the campaign rules for access to and upgrading of magical items. The final and total price of the item (not the cost) is used on the Fame chart to determine whether a caster can apply such an enhancement to a bonded item.