The rules for failing to create a magic item are as follows:
Failing this check means that the item does not function and the materials and time are wasted. Failing this check by 5 or more results in a cursed item.
Question 1: if the item is cursed is the only magic on it the curse or does it also gain the intended properties? (The programmer in me says it would only have the curse since both ifs would be True but then cursed items with other abilities shouldn’t exist, pending an answer to Q2.)
Question 2: does this rule apply to upgrading magic items in the same way? On a failed check would the item totally lose all magical properties or just fail to add the new ones? And would failing by 5 or more add a curse or reduce it to nothing but cursed?