Why not just keep the HDD and buy a M.2 SSD and put that in the M.2 slot? According to the specs you can either put a SATA3 m.2 or PCIe Gen3x4 (nVME) m.2 in it.
Edit: Just noticed you already have an existing SSD you want to use. It should not affect the warrany just keep HDD to swap out if you ever do need to RMA the unit.