I've been thinking about this one.
In Eberron, the Artificer class had a Craft Reserve; a 'buffer' of experience at every level which could be used for crafting. They also gained the ability to drain magic items over time in order to recharge this ability.
Since experience is no longer used for crafting, if I were to port the Artificer over to Pathfinder, I would probably just let them have similar buffer for gp value, allowing them to drain items and use that towards creation of additional items.
If you're not porting the Artificer, or a similar class over, I would still not recommend using a simple spell. Rather, I would recommend the following.
Allow the item's exact magical properties to be transferred over with the full item creation process, offsetting only the gp value, and even then, I might suggest only in part.
For example, a PC has a family heirloom bastard sword. They're very fond of it, and even have the heirloom weapon trait, gaining a +1 trait bonus to attack and damage. As such, they might be resistant to using another weapon in its place, even if there are advantageous bonuses.
Later, the party finds a +2 Fey Bane longsword. There have been a fair amount of fey attacking the party, so the PC would like to gain the benefits of this for his heirloom weapon
The party wizard, using the Craft Arms and Armor feat, both weapons, and the appropriate facilities, is able to transfer the properties from the longsword to the heirloom weapon. This saves on the gold cost of giving the heirloom weapon the enhancements from scratch, but still takes 18 days and a Spellcraft check of 13 (or 18 if the wizard doesn't have Summon Monster I). I might actually consider a higher DC for transferring an item's powers than crafting from scratch. I'm not sure.
Depending on the campaign, you might rule that this still carries some cost (25%-50% of crafting from scratch, for example), but given that it still takes the full allotment of time, and the PC has access to neither weapon during the process, there are definite drawbacks to make the PC carefully consider his options.
You might also wish to consider having the original weapon crumble to dust when the process is complete. This means that when the PC finds yet another weapons whose powers they want for their heirloom, that can't simply swap them back and forth. The old powers can't be transferred out again without permanently destroying the heirloom, and the new powers being transferred in would override the old.