Using the same type of screen as "Request Meld" a player could request a crafter adjust the stats on a piece of gear they have. Adjustments would require:
Materia of matching type/grade (consumed)
Relevant materials (cloth, leather, metal, lumber) of relevant grade (consumed)
The Weapon/Armor to be altered (not consumed)
The crafter would then have a 2-star (or higher?) craft, no HQ possible to complete, materials would be forfeit in the case of failure.

Options would be along the lines of
-1 Determination : +2 Accuracy (Grade 1)
-4 Determination : +8 Accuracy (Grade 2)
-8 Determination : +16 Accuracy (Grade 3)
-17 Determination : +34 Accuracy (Grade 4)

Any piece of armor would be allowed to have 2 adjustments made to it basically making any il90 a BIS piece if you have the adjustments made (an expensive endeavor, and requiring crafters)