[ASnep]

The transponder will have to go off air to make the changes... you can't just change things like symbol rate and modulation type on the fly. Plus the data rate of the new line up has to be correct before going to air with it. 

I'm not how EPG changes work - when they're done (overnight or during the day) and how long they take to react on people's boxes at home - whether a viewer tuned to a service loses it when it is being pointed elsewhere for example. But co-ordinating the two will be tricky, they will want to have the new transmissions up and running for a bit before making the EPG changes, especially where the existing HD service is moving

My guess is that all the LCNs on a transponder will be pointed away from that transponder, then the actual transmission will change. Once that's been on air for a few days the EPG changes will be made to point LCNs to that transponder.

I know they need to take them down at least shortly.

But "a couple of days of testing" would mean that people loose access to regional BBC One on satellite completely, wouldn't it? Because where do they point the LCNs of the transponder that is about to change to?

I'd think the BBC would like to avoid such a situation.