The change itself is rarely the hardest part. The hardest part is usually finding out about it with no warning at all.
As soon as you know a change is coming, say so — even before you have all the details. "Something is different on Thursday, and I will tell you more tomorrow" is a complete, honest sentence, and it buys a child time to adjust rather than being told at the door.
Name the specific thing that is different — "Grandma is staying in your room this weekend, so you'll sleep in the den" — rather than a vague "things will be a bit different." Then say, just as clearly, what is staying the same: the same bedtime, the same breakfast, the same person doing pickup. A child who knows what is not changing can tolerate a lot more of what is.
Asking "is Grandma still coming?" for the fifth time is not a sign the explanation failed. It is a child checking that the fact still holds. Answer it the same way each time, without frustration — the consistency of your answer is doing as much work as the answer itself.
There is no fixed rule. A child who becomes more anxious the longer they have to wait may do better with less lead time and more reassurance closer to the day. Watch your own child and adjust.
Acknowledge it plainly the moment you know: "this is not what we planned, and I know that's hard." Then move straight to what is happening next, concretely. The apology matters less than the next fact.
Use whatever language your child already understands. Some children respond well to the word; others do better with "plan" or "what's happening." Match your child's vocabulary rather than teaching a new one in the moment.
Part of our guide to Visual supports & routines.