If you have a Fusion Drive then you have both a regular HDD which is more readily accessible as well as a blade SSD which is behind the logic board. If you want to replace it with just a dedicated SSD, it's probably recommended to remove both parts of the Fusion Drive, but you should make sure you back up your data first before splitting the Fusion Drive. Alternatively, you could just replace the HDD with the new SSD and then link the two SSDs together as a Fusion Drive, the only downside I can think of is that doing so would create two points of potential failure instead of one. Good luck!