Is ram upgradeable from 8 to 16gb?
title says it all says it all...
No, The RAM has been soldered shut on Apple laptops for the past 3 years at least.
However, if your question was actually slightly different, on purchase you can get the 16GB option. It's a bit ambiguous as to whether you mean pre-purchase or post-purchase. Unfortunately 16GB is the limit. You can't get 32GB options.
I have the same question really, as my RAM has failed in my A1708 Macbook Pro - therefore I need to buy a new logic board to replace it. - So, although I currently have the 8gb RAM logic board in my mac - is there anything stopping my from upgrading to the 16gb logic board, since I am replacing the whole thing anyway?