1) Back up data, wipe drive
2) Sell MBP on eBay
3) Buy new MBP with desired amount of RAM
4) Restore data
Now stop it. You're buying a Mac because you don't like to waste time dicking around with your computer, remember?
My fiancee, who is practically hardware illiterate, just installed brand new RAM and a new SSD into her 4-year-old MacBook. If she had originally purchased it with those upgrades, it would have been $500+ extra at the time of purchase. This week, it cost $150 for the parts on Amazon.
User-serviceable operations that don't require completely replacing the computer are important, and will surely affect many people's buying decisions. You don't get to say that everybody who wants a Mac will never want to waste time dicking around with their computer.
And I do like to waste time dealing with backups and Craigslist scammers? I'm buying a Mac because I want a Unix workstation with a solid UI. Lots of MacBook Pro owners are not terrified of screwdrivers.
Ha. Best of luck, friend.
Not to mention if you hit your HD limit after a year the bigger drives are going to be drastically cheaper.
The same goes for memory, hard drive, and other parts of the computer.