Point is, on what firmware you are with curent hard drive, after replacing drive, you will be on same firmware, or you can update to newer.
You cannot download some random firmware version, replace drive and than be on that fw version, you need the same of newer.
Maybe one day we'll have flasher, but if we have that level of acess to downgrade firmware, we'll probaby dont need it anyway, so my guess is that there will be no downgrade option in near future