It is possible to downgrade from Smart Navigation to Orca Plus, but this depends on how you subscribed to Smart Navigation in the first place.
Subscriptions via Google Play or Apple App store
Your Smart Navigation subscription will last until its original end date, so you will not receive a refund. When the original end date passes, it will switch to Orca Plus, and you will pay the Orca Plus subscription price instead.
Subscriptions directly from Orca
Your Smart Navigation will instantly downgrade to Orca Plus. However, you will not receive a refund. Instead, you will obtain a prorated credit on your account, which can be used to deduct from future subscription charges.