Unsurprising, since really they're under no obligation to give you anything other than a cargo van just how you're under no obligation to rent from them. That being, though, I'd push back on them for their BS. If they don't the cargo van and can't get one, then I'd be pushing for a minivan or pick up truck to "fit" what you need to "transport" at your cargo van rate. It's on them that they can't get specifically a cargo van and unfair to try and force you to rebook at a much higher rate. They're still under no obligation to do this, though, but you can try. They may have figured out you just booked the cheapest class given how long your rental period is. Though you can counter that you're moving to a new place and want to take your time.

You can also try complaining on twitter but based on some cursory searches on FT it seems like they take days to respond and don't have as much pull as before. Maybe a call to the reservations line and an escalation to a supervisor can get them to reserve you a truck or minivan at your current rate, but that'll also be tough. Good luck!