1. The countdown is just a projection based on usage, if it says 17 days and you use it less than is typical it will absolutely go longer, and it is a guess based on usage, never a hard number.
2. And yeah, it's going to be expensive. The devices are programmed to use whatever was loaded first. So they were on a Metered contract and got a Metered cartridge to put in, which made it a metered machine. The Only way to convert it back is with a special cartridge that resets the chip inside, and then load a non-metered toner to convert it to "Sold". A Xerox tech can do this, it is not free of course.
The 3rd option is probably more expensive, it would be to go through Xerox and get a contract on the machine, but that typically means yo have to pay them to inspect the printer to make sure it is in good shape, not stolen etc etc.
Now, everything stated here is generic, because it is a Worldwide forum, and I mainly support American products, while living in Canada, so everything here is just "Generally correct", it may not be exact in some areas, but it will certainly be similar at the least.
There may be 3rd party cartridges that can work (programmed as metered) but I don't know, I haven't looked, and they would not allowed to be posted here if there were.
I have recently bought a Workcentre 7830 (second hanbd) and there is a day countdown which is running (but the supply contract is no longer valid), so my machine is in Metered mode. My question is, when the countdown will be over, do I could still use the machine ? (with normal supplies) ? or does it ask me a code or block the copier ?
I hope that someone could reply me ...