I'm sorry if this has already been answered dozens of times but I couldn't find it.
My barbarian as a +1 Greatsword and I'd like to see about getting that bumped up to +2. This weapon already cost her 2,350 gp (bonus squared x 2,000 + masterwork).
So my question is, would it cost 8,000 gp to have it enhanced further to +2 or is there a discount since the weapon is already magical?
Adding New Abilities
Sometimes, lack of funds or time make it impossible for a magic item crafter to create the desired item from scratch. Fortunately, it is possible to enhance or build upon an existing magic item. Only time, gold, and the various prerequisites required of the new ability to be added to the magic item restrict the type of additional powers one can place.
The cost to add additional abilities to an item is the same as if the item was not magical, less the value of the original item. Thus, a +1 longsword can be made into a +2 vorpal longsword, with the cost to create it being equal to that of a +2 vorpal sword minus the cost of a +1 longsword.
If the item is one that occupies a specific place on a character's body, the cost of adding any additional ability to that item increases by 50%. For example, if a character adds the power to confer invisibility to her ring of protection +2, the cost of adding this ability is the same as for creating a ring of invisibility multiplied by 1.5.
I'm not sure what the official ruling is, but in every game I've played, the DM allowed items to be upgraded by paying the difference.
So, your +1 Greatsword is currently 2350 gp, and a +2 Greatsword would be 8350 gp, so you pay 6000 gp (and give the crafter your Greatsword while they work on it) to change your +1 to a +2.
Thanks everyone, appreciate the info. Glad to see that it's just the difference in cost and not losing the original amount.