In the MacBook Pro Retina, the Flash Storage device is NOT permanently soldered to the logic board, and yes, it can be upgraded. I found an original Apple 512 GB flash storage drive on eBay. http://www.ebay.com/itm/200858649301
I'm going to try to fill in the missing link here -- Flash Storage and Flash Memory are indeed the same, and it is just like a Solid State Hard Drive, but with a different shape and connector and no enclosure. Solid State Hard Drives, USB thumb drives, and the new Flash Storage devices are all composed of NAND or NOR gate Flash memory technology, as are Compact Flash cards and SD cards for cameras and phones. This technology is unique from other types of memory such as battery-powered Static RAM or Dynamic RAM. Flash memory is NON-VOLATILE permanent data storage and can be removed and connected to another computer and the data on it will persist without power, just like a typical "hard drive" or "thumb drive", making it a suitable replacement for magnetic media such as tapes and hard drives. What differentiates a CF card from an SSD from a new Flash Storage Drive is the available sizes of memory chips, the speed of operation, and the interconnect port with the computer. But at the core, they all use NAND or NOR gates.
On the other hand, there is VOLATILE memory, known as RAM (Random Access Memory) or DRAM (Dynamic RAM). All modern computers in the last 30 or so years use volatile DRAM memory to temporarily store data and instructions for the CPU to access and work on. The fundamental architecture of DRAM is a capacitive (dynamic) memory storage element which loses its charge state with time and has to be refreshed by the memory controller. The most widely adopted and successful system architecture of DRAM in computers today is SDRAM (Synchronous DRAM). The MacBook Air and MacBook Pro Retina both have SDRAM which is permanently soldered to the logic board. The type of RAM in this generation of computers happens to be called DDR3L 1600MHz or PC3-12800.
Clear as mud?