[Stuart]

Could it not all happen on the same day, presumably 23rd January?

I'm sure they can create the UK wide 'BBC One SD' as soon as the sub-opts in Oxford/Cambridge have ended in a couple of weeks.

However, I don't think any organisation would want to do a major change all at once, when they can stagger the process over a number of weeks/months.

It's only going to be noticed in each individual English Region, so viewers won't know that the neighbouring region switched the week or month before.