The machine that could do that is 15,000 USD or more. Then the tricky part is having the computer recognize the new RAM. I would say impossible to costing a fortune (at least thousands).
There is also perhaps only a 50/50 chance the motherboard will still work after the removal and replacement of RAM. Pulling chips off these boards and soldering them back on with the eco friendly (high temp and tricky) solder is not for the timid. Any chance of success requires highly skilled techs and sophisticated equipment. It may be more cost effective to replace the entire board.
In other words, you would pay maybe $1000 up front and have a 50% chance you lose your entire board (and the money).
Knowing that the RAM is not upgradable, I would shell out the $200 for the maximal configuration upfront, and not worry about the future. I don't see 16 GB RAM becoming a bottleneck anytime soon. The GPU and video RAM are more constrained.
I have a soldering iron send me your MacBook pro Retina and ill take care of it for $20.00 no guarantees it will work though, Or just buy one with 16gb and you'll be fine for many many years
Seriously though, if you bought one with 8 GB and a year or two later decided you wanted 16 GB, it would be cheaper by far to sell it and buy the latest & greatest (or even buy a used 16 GB).