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That's actually the first good theory i've heard. SO maybe it could be the case that they change every 4-5 days?
They would have to give up on the rank progress they have made which will impact the daily focus chests which in the end represent more potential than the season chests. So it's not certain that the switch will be the case. BUT if they can quickly get their rank back up to a similar number on the new account then perhaps this could be the case.

How much harder is it to get chests after your 50th chest?