I don't know if there's an official answer, but I've read posts where people have stayed between two dining periods. I don't think they'd ask you to leave. I think it tends to happen more between lunch and dinner though.
According to the Disney website, breakfast ends at 10:30 am and lunch begins at 11:30 am. I would guess this means that they continue to put out breakfast items until a little before 11:30 am, and then they start to bring out the lunch items.
We have been there at CP for the last breakfast seating a couple of times and when they started to change the buffet over to lunch they did allow us to go back up and get some of the lunch food. Don't know if we were just lucky or if this has changed but that was our experience in the past.
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