A complicating factor was that I was setting this up beforehand, as I would be on holiday when the channel would tick over to its rebranded version. So I had to set up the lookup table in such a way that it wouldn't air any of the new stuff until it was time.
Basically: I copied the lookup table, set an expiry date on the top half and modified the bottom half to use the new assets from 6am on July 24th.
The lookup table existed of more than 1600 rules at some point. The horror. But: it worked!