Hello!
Do you have to make a Spellcraft check when adding new abilities to/upgrading magic items?
For example, when upgrading an Amulet of Mighty Fists from +1 to +1 Flaming. Do you have to make a Spellcraft check, or do you just have to meet the construction requirements for the new abilities, pay the additional gold, and spend the required time (that's a mouthful, so in the future when I say "construction requirements", assume I also include the gold and time requirements)?
If you just have to meet the construction requirements (no Spellcraft check required), does that mean it's impossible to upgrade magic items if you don't meet all the new construction requirements? When crafting brand new items you can just increase the DC for requirements you don't meet, so it'd be a pity if you were unable to do the same when upgrading items.
You do need to make the crafting check (usually with Spellcraft), so you can do the same "skip requirement for +5 DC" bit.
You also have the same risk of making the item cursed if you fail the check badly (but this probably doesn't matter because you're taking 10 for a sure thing, right?).