Whenever the upgrading should happen:
Set Integer A = 1 to 6
Check if slot Integer A contains Ore
Check if slot Integer A contains Broadsword
If both true:
Set RANDOM = Random number beteen 1 and 100
If RANDOM is less then or equal to xx (Your percentage), then do:
Remove Ore, remove Broadsword, and add Broadsword+
I got no WE here, so I can't give it to you in correct triggers, but all these commands are there.