It can be done on A1502 models, but it requires skills, equipment and the correct 16GB RAM. This repair isn't for the faint of heart and can kill your machine. There's a few manufacturers who make the chips and they're not proprietary. They can be either salvaged from another MacBook or from a inline memory module. The process involves desoldering the existing RAM chips using hot air and resoldering in the higher capacity ones. Depending on the manufacturer, you will also need to either remove or move a resistor location. There's a chart (somewhere online) listing the different RAM manufacturer and the correct resistor configuration.