The Adding New Abilities paragraph in the magic items rules, gives the example of turning a +1 longsword, to a flaming +1 longsword, costing only the difference between a plain +1 weapon and a +1 flaming one.
Now, since any caster capable of creating a flaming weapon also meets the prerequisites for creating a +1 flaming weapon, it is not clear to me what exactly are the prerequisites for adding an ability.
For example: a level 5 wizard with 'Craft Magic Arms and Armor' and access to the Fireball spell meets all the prerequisites of the flaming special ability. He does not, however, meet the prerequisites of the vorpal ability, or those of a +4 weapon.
So, can that wizard take a +4 vorpal weapon and turn it into a +4 flaming vorpal weapon?
Please stick to the RAW in your answer.
p.s. I'm aware of the option to craft an item at a higher DC without meeting all the prerequisites, let's ignore that for simplicity (also, since even in that case it is important to know what is the complete list of prerequisites in order to calculate the DC, the question remains relevant for that case as well).