Surface Pro - HardDrive replacement

Does anyone know/think it will be possible to switch out the harddrives on a surface pro? I imagine that they are using the really thin SSD's as found in ultrabooks, but 128gb as a laptop replacement for me won't cut it. My main laptop now uses 150gb, and that's what I really need. So I'm left with a simple question of whether or not I'll be able to fit in a 256gb.