I'm not in the US so I don't know how it works there, but over here the stations keep the same numbers. If the frequency needs to change for whatever reason, then they somehow signal the receiver to update the frequency. If it was "channel 9" before, then it'll still be "channel 9" after the change. Does it not work like that in the US?
I honestly know zero about the UK's broadcast system, but in the US's analog days, channel 9 broadcast over channel 9, which lies in the VHF band. This number was important not only for reception, but for station branding and Neilsen's ratings systems. With the digital switch, most stations were assigned a UHF channel # (I think that's channels 13 & up). So channel 9 is now broadcast on digital channel, say, 45. Most stations are embedding
PSIP information into the signal so when your ATSC tuner or convertor box sees channel 45, it displays it on channel 9. When you're flipping channels, you'll still hit channel 9, but the actual signal will be coming over channel 45. Confusing, I know. The mian reason for this as I can see is to retain the decades of station branding that's been beat into viewers heads...
Some convertor boxes aren't great at recognizing this PSIP info, so we tell people who can't find us on 9, to try tuning in channel 45. It should then see channel 45, and recognize it as channel 9.
EDIT-Just saw that your in NZ and not UK. Sorry, but I don't know anything about Kiwi broadcast standards either, but I hope my response helped.