I tried to change my contract from the 250Mbit to 1000Mbit sometime last year, I got a new router that supports the new upgraded speed, but the device was not working as the old contract was still running. I had a call with a Vodafone agent (they called me, I didn't call them) and they confirmed that the new router is not working because I'm still running the old contract. Up till today my contract still says 250Mbit instead of 1GBit and the new router still refuses to work.
However, in my January bill, I have been charged a hefty amount for not returning the old HomeBox, which I can't return since the new router doesn't work yet and the new contract hasn't started.
Additionally, I received no instructions on how to return the old router, though this does not matter as I couldn't have returned it without the new contract being activated.