Personally, I would allow it, but without any price break -- ie, it still costs the same as getting a new +5 bastard sword. There's no mechanical effect, but it just helps the feel of the game to be able to think of it as the same sword.
I'd allow it, probably just making the crafter pay the Masterwork cost/time again and maybe an ad hoc fraction of the XP cost. The materials have already been spent. But I'd certainly tien in an adventuer into this.
I wouldn't allow it. Just because someone isn't proficient in the +5 Orc Double Axe of Smiting All who Stand In My Path, doesn't mean they can reforge it into two +5 Axes of Smiting All who Stand In My Path. No way. You want to use the weapon...become proficient in it or sell it to someone who can use it.
Although it could be argued that this wasn't technically reforging, I believe that some DMs would allow a wish to turn a +5 longsword into a +5 melee weapon of another sort. . . .