Hi there,
an ntl installer has provided me with a crossover dongle when he swapped over the STB before I was swapped over to the stand alone modem.
Although I'm not a tech I have a feeling that the first tech was correct, your account is tied to your equipment, whenever an STB/modem swap is done then they need to change the account information to point to the new equipment.
For some unknown reason I used to chew up the old PACE boxes like they were going out of fashion (I'm very technologically aware and take care of my equipment but still killed more STB's than I'd care to remember!

) so I'm used to waiting a little while after the installer has left for the account to be flipped over to the new stuff.
I feel that the person at fault here is in fact the installer, not the tech support advisor.
Cheers.