It's likely possible, but would require special tools like vacuum tweezers, SMD soldering equipment etc. Apparently some folks in Asia like to desolder the baseband chip in iPhones and modify it and solder it back in.
If you are asking if its possible to DIY it, i'd say no unless you have access to a SMD prototyping lab and have the appropriate skills.
I'd like to know what you are putting on the phone that makes you absolutely need 64 GB, but it's not really any of my business so