SQLQuokkaSQLQuokkaDropbox stores every subscription plan change as a row in `plan_changes`. The most recent change per user (by `changed_at`) is that user's current plan. Write an SQL query that returns, for every user, `user_id`, `current_plan` and `previous_plan` (the plan immediately before the current one, or NULL if the user has only ever had one plan). Order the results by `user_id` ascending.
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