This is really 2 questions, but here I go:
1. If someone already has a +1 sword can it be upgraded to a +2 and if so, would they pay the full 8,000 or 8,000 minus the 2,000 for it already being +1?
2. If someone has a +1 sword and they want to add flaming, do they pay 2,000 since it's a +1...