I recently purchased a Dell Inspiron 8200 with 8Gb of memory (DDR4, 2660 Mhz), which turns out not to be enough. There are two memory slots. I intend to upgrade to 16Gb, (two 8Gb sticks). If later I want to upgrade further still, would I be able to use one of the existing sticks, having, let's say, one of 16Gb and one of 8Gb for a total of 24Gb, or do both sticks have to be the same size?