Continuous magic items, like +2 to strenght, can be improved to +4 to strenght, paying the difference between the cost of the two items.
I would like to extend that possibility to magic items that have some effect usable a number of times per day keyed off caster level, such as eternal wands, making it possible to pay the difference between an item with x caster level and x+1 caster level in order to improve it.
This would make it so crafters that don't want to abuse their crafting ability don't have to sell older equipment for more than they bought it, profiting more than intended. Instead of profiting when they want to upgrade an item, they just pay the difference.
Example of what I'm talking about: some magic item with caster level 5 costs 1kgp to buy. A dedicated crafter can craft for 30% of the cost, so he can craft it for 300gp. When the crafter gains another level, he wants the same spell, but with caster level 6, which would cost 1.2kgp, but he can craft it for 360gp. Normally, he would have to sell his old item for 500gp and craft the new one for 360gp, profiting 140gp in the process. What I'm requesting is the ability to just pay the 60gp (360-300) to improve it.