I would, at the very least, require a feat to let someone brace as an immediate action (swift wouldn't work since you can't take swift actions outside of your turn) instead of a prepared action.
Changing the weapon from scythe mode into spear mode as a swift action might be OK (depending on what the weapon stats would be), but the part where you get to a prepared action for free is too good even for an exotic weapon.
And I'd make the feat generally available: "Reactionary Bracing". Works with all weapons with the brace ability and you get to set the weapon against a charge as an attack of opportunity (that also counts as an immediate action - so you can't do it more than once per round). Don't know about the requirements, but they shouldn't be trivial - after all, you're getting double damage out of this.
I suspect that this is one of those weapons that will need a lot of build up and not something that will get a strong raw power advantage. Near as I can tell, just slap brace on the scythe and make this new scythe exotic. No real need to declare a swift-action to use it because brace only works on a readied attack anyway. I would be wary of giving it a swift-action auto-brace because that is essentially a fifth level specialist fighter class feature, and even that is limited to only a few times per day. My advice to make it cooler is to play it up and describe it. Longsword means a lot of things, some cool and some not so cool, it is up to the DM making junk up to put it in the first catagory.
Has the player come up with a plausible mechanism for having a button on the handle straighten and extend a big hunk of steel at the end of a pole?
Is it magic?
Just off the top of my head, this seems to be a power that would require some sort of magic to accomplish (thereby making it a magic weapon with the consequent cost). As far as being able to do it when someone else charges him? That's a tougher call. It would have to be an immediate action. I would consider making this a feat.
