I'm surprised to see no Single-Board Computer including Raspberry PI has memory modules for allow for flexible amount of memory (e.g. this review). And I have not found such discussions via web search, for PI I've found only that post on PI site Improve RAM with a laptop's memory module:
No, not possible. The Pi does not have a memory socket of any description - memory is soldered to the board.
No comments why, no comments whether it is in plans. I understand form factor is not large and it is densely packed already, but as amount of DRAM is important to many (IMO), why none of producers make at least one model with a removable SO-DIMM module? Any particular technical challenges?