Spellcraft DC when improving magic items

[mehs]

Okay so say you have a magic item or some **** that is able to cast a number of spells per day or such (staff, wondrous item, etc), and you are upgrading it to be able to cast more different types of spells per day. What would the spellcraft dc be affected by? Say that the majority at least of the spells are ones that you can't cast on your own and you dont have a helper who can for all of them that you are adding or that is already in the item.
Would you be taking a penalty for all spells in the item already as well as the new ones that you cant cast? could you use the item itself as the thing helping you cast spells while making the magic item for the spells already in the magic item?

Spell A already in magic item, cant cast
Spell B already in magic item, can cast
Spell C not in magic item, cant cast
Spell D not in magic item can cast
how would each of these above affect spellcraft check dc?

[Crake]

If you're improving an existing magic item, the final product's prerequisites will still all require the spells and whatnot that are already inside the magic item, so A and C would increase the DC by 5, while B and D wouldn't affect the DC.