The script is designed to stop people from fucking around and abusing it and getting statistical values over +5, or making overwhelmingly stupid combinations of stats by taking a +5 AC item with bonus spells and concentration bonus applied and adding 6 charisma to it to free up a ring slot for something else or whatever else.
Essentially, the forge will wipe previous AC from it and start again from 1, then 2, then 3, etc. It was designed for crap items to be used on them; basic swords and weapons and what have you. It was not implimented to give you a reason to get +9 on your +5 wizard amulets, or +5 AC on your +4 wizard amulets. It was meant as a transition period to give you something to work with whilst leveling up before you get faction gear. Orizvi is apparently working on fixing it so that you can do what you want, however, I believe.
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