The 16GB and 32GB eMMC chips have identical electrical interface and ball assignments as defined by JEDEC eMMC 4.41 specification. Thus a 32GB can fit on a board that originally had a 16GB chip. However, the original 16GB chips may contain factory configuration and boot programs that have to be also programmed into the replacement chip. The OEM usually does this with a special equipment and software to initalize the virgin eMMC chips (probably via a JTAG port, or a special boot procedure via USB).