My google fu must be weak, I recently picked up an old p64 and was entertaining the idea of converting it to .380. As far I can tell, its a "simple" barrel change and nothing else. Could it really be that simple?

Supposedly.  It's based off the Walther PPK series so pressing the barrel out would allow you to swap them.  There were .380 conversion barrels made for the PA-63 and the P64 at one time but I'm not sure if you can still find them.  

I found this with a little searching (out of stock of course): http://www.the-dealer-warehouse.com/Polish-P64-Barrel-380.html  You could try calling them.

You can shoot 380 out of a 9x18 just fine. Slightly inaccurate but probably not enough for a normal shooter to notice. If you do it a lot it'll wear out the barrel faster due to gas blow-by, but new barrels are cheap. There are zero safety issues from doing so.



Why do it though? 9x18 is much cheaper and there are great defensive loads from Buffalo Bore and Hornady for it.



If recoil is the issue swap out the recoil spring for the heaviest Wolff replacement (it'll still run reliably with all ammo I've used in mine) and enjoy one of the softest-shooting 9x18 setups there is. Just be sure to use a dremel to grind the front of the new spring flat to resemble the old spring, or it won't reassemble right.

 Just because you can do something doesn't make a good idea.  The 9x18 uses a bullet that is .364" in diameter and the 9x17/.380 bullet is .355".  They seat off of the case neck, not the rim like say a .38/.357.

I agree.  The 9mm Mak (9x18mm) and the .380 ACP/9mm Kurz (9x17mm) both head space on the case mouth.  

Consequently, when you shoot a 9x17mm in a 9x18mm chamber you've got 1mm of excess headspace, which means the cartridge is really head spacing on the rear face of the extractor after the firing pin drives the cartridge forward.  

Which weight spring do you have? Because i like the idea of swapping out the stock spring for something stiffer.

You can shoot a .380 out of a 9mm Mak just fine...right up until you blow the extractor. This could be considered a safety issue.
You aren't going to hit shit with it, either, the bullet will be wobbling down the bore like a Brown Bess Musket. It's entirely possible it will tumble upon leaving the bore, too.
Barrels are cheap but pressing them in and out of the frame is a major PITA.
There is no reason whatsoever to shoot the .380 in a Makarov chamber. If you want a .380, buy one.

Why convert it?  I'd just as soon shoot cheap or surplus 9x18 out of my two P64's than expensive 380 ACP.  And premium self defense ammo is available if you want it.