Chosen Solution
Can one upgrade the memory from 8g to 16g by replacing the logic board that has the increased memory installed?
You can do that, although beware of spending more than the upgrade is worth to you.
You certainly could, for a price, you could then sell your board in turn to offset the cost. Just bear in mind that some other parts are paired to the logic board, most notably Touch ID. So swapping the board could break some of this functionality, unless the touch ID module that was paired to that board is also swapped to your machine. Also be aware that Serial Number is tied to the logic board. So buying a used logic board would also change your serial number. That is probably not a huge deal, but it could mean some additional steps if you utilize any licensed software.