Look if and when it turns up, stick it in the corner as you say, if your existing hub gets disabled, then connect the Hub 5 and immediately put it into modem mode and there shouldn't be any issues.
Ahh, well that's what I thought, but since the Hub 5 arrived and went straight into modem mode my broadband reliability has been ****ing appalling. Yet another loss of connection in work time today (only 20 minutes this time, yippee!), but the past couple of weeks has seen roughly hour long connection losses every other day.
I can assure @Tweedriver that what happens is that once the Hub 5 arrives, if you put it in the corner unused, your existing hub will sooner or later be deactivated - took about two weeks with mine, but that happens without you doing anything. Plugged the Hub 5 in, and mine worked straight off without activation...well, worked except for the bloody dropouts.
Best bet (if you have no problems at the moment) is to phone up as soon as it arrives, and try and explain that you were sent another hub that you didn't ask for, can they cancel the Hub 5 because you don't want to be using an untried product - unless of course, you are willing to give it a go. Make sure the agent understands what you're asking, and confirms they can action it, as given VM's low calibre customer service there's ample potential for things to be done incorrectly.
As for @BMWCHUBB, what benefit are you expecting from a Hub 5? Fair enough if you want to give it a go and accept the risk of poor reliability, but is that really the case?