Hi, yes I think that's correct. If you want to utilize SAS/SATA drives, you must choose a backplane that supports their use. If you decide later to upgrade to NVMe drives, you would need to replace the backplane, drives, and cables for signal and data. This is because NVMe drives utilize a different interface than SATA/SAS drives, and require different hardware components to function properly. Regarding your concern about the motherboard, it is hard to say for me because I don't have decent data about that. Occasionally, the motherboard is universal and can be used with different backplanes.  As I know mostly the only components that need to be replaced are the backplane, cables, and drives. I would expect this should be enough to transition to a different drive type. 
Hello, I do not understand the storage options, please clarify. As far as I understand the U.2 ports must be backwards compatible with SATA/SAS drives, but the usage manuals for Dell servers give very vague description, for example:   24 drives system Up to 24 2.5 inch NVMe drives in bay 1 (slots 0 to 11) and bay 2 (slots 0 to 11) OR Up to 16 2.5 inch (SAS, SATA or Nearline SAS) front accessible drives in slots 0 to 15Up to 12 2.5 inch NVMe drives in 8 universal slots 16 to 23 OR Up to 8 2.5 inch (SAS, SATA or Nearline SAS) in 8 universal slots (bay 1 slot 0 to 7) and 16 2.5 inch NVME drives in bay 1 (slots 8 to 11) and bay 2 (slot 0 to 11)   Do I understand correctly that Dell NVMe backplanes are NOT backwards compatible with SAS/SATA? So for example if I buy a server with "Up to 24 2.5 inch NVMe drives in bay 1 (slots 0 to 11) and bay 2 (slots 0 to 11)" backplane I will have to use ONLY NVMe drives, not SAS/SATA? Or if I buy a server with "Up to 8 2.5 inch (SAS, SATA or Nearline SAS) in 8 universal slots (bay 1 slot 0 to 7) and 16 2.5 inch NVME drives in bay 1 (slots 8 to 11) and bay 2 (slot 0 to 11)" I will NOT be able to use SAS/SATA drives in bay 1 slots 8 to 11 and bay 2 slots 0 to 11? My point is: I want to use exclusively SAS/SATA drives for now, but still want an option to upgrade to NVMe drives in the future without replacing the backplane.
2.5" NVMe backwards compatible with SAS/SATA?