Do you mean change your delivery date or just the items ? sorry I'm not arguing that this happens but surely it's all linked to dates and offers are either valid on the delivery date or not?
The wording of the offers is "when you order by " not "delivered by".
So my next order is 7 Jan. Pepsi Max is "Now £7, when you order by 05/01/2021". So if I put it in my basket today, I should get it at offer price for my 7 Jan delivery. If I wait til 6 Jan to add it (assuming my basket is still editable at that point!), I'll have to pay full price.
That's how is has always worked, until recently at least. Just before the offers expire, I go through adding them to all of my upcoming orders, just in case (I then remove them nearer the time if I don't want them). But as I said in my PP - I think that last month, some of the offers in my basket had gone back to full price, so they may have changed it now.