Short answer: No.
Explanation: Such changes would have influence on all existing registrations. So you would have to cancel all registrations for participation before you can change the currency or VAT rate, i.e., you must set the currency before you open the registration phase. Therefore, these settings are locked after persons have registered as participants.
Long answer: If such changes are necessary after the registration phase has been opened, please contact us and explain the new requirements.
Sometimes we find a solution to deal with such problems without cancelling all registrations.