Microsoft’s Surface Pro is different from current ARM or Atom based tablet, because it supports Windows 7 software installing and integrates an industry standard mSATA Micron C400 SSD. According to iFixit, you are able to technically upgrade the SSD based drives inside to much larger mSATA SSDs. Someone has successfully upgraded the 64GB hard drive to a 240GB Intel 525. That broke the tablet but cool upgrade.
Warning: It is Dangerous. The tablet’s touchscreen connector contacts glued on two sides are made of a conductive powder and the one on back side is glued onto the Surface case with strong adhesive. So if you disassemble the Surface pro, the touchscreen functionality would be destroyed and the device become brick.
After opening surface pro by following ifixit Teardowns Guide and changing internal storage to 256gb ssd or more, you might need to probably use a plastic tool to scrape off the old adhesive and reclose it by foam double sided tape.