The elven greatbow is normally considered a +2 composite longbow, but in the hands of an elf it becomes a +5 unerring accuracy longbow (+11 total upgrade) with other special properties.
Now, while we all agree that the price for any upgrade must be multiplied ×10 compared to the normal formula (upgrade2 × 2000) since it falls under the epic rules due to its price, I'm not sure how much would I need to pay to further upgrade it.
For example, say I want to add a +1 to the bow, so that it becomes +3 (or +6 in the hands of an elf), would I need to pay the price to bring it from +11 to +12 (460,000) or from +2 to +3 (10,000×10=100,000, as per the epic magic items rules)?
On second thought, would this be actually a valid upgrade? Unlike a bane weapon, which states that the enhancement bonus increases by +2 (so it doesn't matter what is the starting enhancement bonus), here it says it becomes a +5 bow, would that mean that any upgrade I put on the bow get overwritten when wielded by an elf?
I'd like to be as RAW as possible here.