How and at what stage in the application process do they force? Could it be because there are more locals applying than the available seats for them?
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I think almost anything about your application isn’t permanent until you submit it.
There are always many times more applications than available spots day OR boarding at Andover. I imagine the change in policy had to do with problems with people wanting to change from day to boarding or vice versa previously. But I have no connection with Andover so cannot speak to their change specifically.