For a downgrade the [U]reimbursement[U] (EU261 is specific in its language for a downgrade there is reimbursement not compensation) is a % of the cost of the sector downgraded and not the entire ticket excluding the government taxes and airport fees. There was a court ruling last year that clarified this point.
If you paid in miles then the reimbursement will be in miles. The airline has no obligation to offer you the cash equivalent of the cost of buying those miles for example. It might not be as much as you think it is!