Maybe this helps OP understand it better:
This is just example.
Item database:
Mofifier number 1: "Leech applies instantly on critical strike"
Modifier number 2: Life(item stores value)
Now, lets say unique item drops and rolls mods:
Example gloves unique vaal gauntlets
Leech applies instantly on critical strike
Life 60-100 <--- Now this value is stored on the item and they cant change the value.
Lets say the item rolled value 70.
Now, if they change something, they cant change that 70. Instead, they can change whats on the database. They could change "Life" to "Mana" but that would mean every item that had life, has mana now instead. (bad example, since mana already exists in database, but lets pretend it doesnt).
So, they could change that "leech applies instantly on critical strike" to "Fish is caught on critical strike" and all items that had that old mod, would have the new one.
After they changed the database, new item would look like:
Example gloves unique vaal gauntlets
Fish is caught on critical strike
Mana 70