[Newsroom]

What time is it changing over? C was active to 1625. Bulletin repeated at 2700. But at 1800 all from E, then another story at 1839, then rebroadcast if bbc1 from E with Rita Chajrabati. Then Luquesa returned at 1855 also in E.

I assume they are getting studio C ready but when does the new system begin? Thanks.

I think you're a bit confused about what's happening tomorrow. 
All that's happening is the joint daytime output is from E (apart from the 1pm hour when World moves to C), evenings (7 - 10pm), overnights (2 - 5am) & World early mornings (5 - 9am) from C.
World & the NC have been joint all weekend, there's nothing to "change over" and no "new system" is starting tomorrow.

Other than there'll be no way the BBC can provide UK specific Breaking News for the month. Technically impossible by all accounts .