Just a few minutes ago, sort of went through this. My cable provider upped my speed again - for no extra charge (how DARE they! Such horrible money-grubbing Capitalist Pigs...oh, wait..). Of course, the cable modem I purchased some 6 years ago to stop paying their rental fee (paid for itself in the first year) is no longer compatible with the higher speed (my modem is DOCSIS 3.0, they are now using DOCSIS 3.1 for higher speeds), so I have to buy a new one. So I could buy a new one, or a certified refurbished one (not stating who refurbished it - but it was NOT the manufacturer) and save a whole $18. Nope, I'll take a new one, thank you. Refurb also only gets a 9- day warranty, new has a 2 year warranty.
So unless there is some fine print in the warranty that says they can replace with clearly used equipment, I would complain. They are treating you like someone who is leasing, not an owner. If you were leasing, no issue, they are under no obligation to send out equipment other than that is can provide the service level you are paying for. If it works but some previous user let their kid draw circles on the case in a permanent marker, well, so be it. But since this is a purchased product under warranty, I think you deserve more. You got a fair quality used product, not a refurbished one. To me, a refurbished unit is one that is tested to prove the electronics all works, and it LOOKS like a new product, but may be missing the original packaging, manuals, and/or cables.