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Item IDs are structured logically
XXXYYZ
XXX = Item Type (bows = 500 for example)
YY = Item Level (33 being a lvl 130 bow in this case)
Z = Item Quality (1-9 9 being super)
So when upgrading item level you try to increase level by 1 until you find a valid new item level. Check item type against the new item ID to ensure you didn't roll it into a different item and then you're fine. Add a santiy check to loop a maximum of ~5 times and you're done.
Down leveling is just setting level portion of the ID to the lowest item level that item type supports (usually 0 or 1). Personally I match equipment slot ID with a minimum level and use that when reborning.