We changed a few months back, about an hour before the engineer was due I removed the old one and connected the new. Engineer was in house for 20 minutes, changed the wall socket and tested it (he had no cables to run, I'd already done it) When he left he was going to the Box in the street, 'phoned me about 15 mins later said push the reset button, that was it, all done.
*For the hour it was connected before he arrived I was getting the old service through it same as the old Hub!*
I should have said it arrived in the Post about a week before fitting.