An error budget spent deliberately is the point of having one

A budget that is never spent means the objective is too low, and a budget that is exhausted every month means it is too high or the system is unhealthy.

99.9% over 30 days = 43m 12s of budget.

  spent on:  a deliberate migration cutover      18m
             an incident                          9m
             a deploy that was rolled back        4m
  remaining:                                     12m

and the policy, agreed in advance:
  budget exhausted → no non-essential deploys until the
  window resets. reliability work takes priority.

The policy is what makes the budget mean anything, and it has to be agreed before it is first exhausted rather than negotiated during. Spending it on a planned migration is the healthiest possible use — it is a scheduled, understood risk taken because the budget existed to cover it.