Here's the rules text in question:
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.
I'm not sure if this allows players to, by RAW, upgrade specific magical items. For instance, a suit of mistmail costs 2250 (market price), and is a +1 chain shirt. Could one pay 3000 more to upgrade it to a +2 chain shirt (1500 more if crafting it oneself)? Or would that require a houserule?
There's debate on it. The rules are silent on this specific topic, but they do say you can upgrade "magical items". I personally see them no different from "normal" magical items, and fervently believe that by saying you can't enhance them you're saying that there are two different types of magical items in the world, which is preposterous.
Figure out the extra cost added on for the magical abilities, and then upgrade the item to what you want. Then add on the extra cost. In effect, the extra cost *is* the cost of the special abilities.
Specific items technically require a house rule to update. Its an incredibly common house-rule though.
There's debate on it. The rules are silent on this specific topic, but they do say you can upgrade "magical items". I personally see them no different from "normal" magical items, and fervently believe that by saying you can't enhance them you're saying that there are two different types of magical items in the world, which is preposterous.
Figure out the extra cost added on for the magical abilities, and then upgrade the item to what you want. Then add on the extra cost. In effect, the extra cost *is* the cost of the special abilities.
This is roughly how it seems to work out from my perspective as well.
However, some items may have additional "+x" abilities. For instance, while a Dwarven Thrower is only a +2 warhammer, it's a +3 returning warhammer in the hands of a dwarf. It could very well be that "additional +1 and returning in the hands of a dwarf" is calculated as a +1 bonus on this item, and the rest of its abilities are a fixed GP cost. After all, every other "add an additional +x to the enhancment bonus in these specific circumstances" enhancements are always priced as enhancement bonuses not fixed GP bonuses, and I think that's for balance purposes.
The pricing ambiguity makes me think this might not be RAW possible in ambiguous cases, or maybe not at all with specific weapons and armors.
I can't find it but i'll paraphrase it from one of the devs as best i can.
Most custom items are hard to upgrade specifically because its hard to tell what the upgrade is worth. There's no formula on the custom items to backtrack how much of the value is from the custom and how much is from the +'s. IE, is the celestial quality of armor with X amount of gold peices? Is it worth a certain + equivilant?