The website is not transparent on this, but according to their chatbot, legacy members remain subscription-less.

The chatbot said:

Oura Ring 2 owners who upgraded to a Oura Ring 3 when it launched were upgraded to a Lifetime Membership. Currently, there's no option to purchase or upgrade to a Lifetime Membership. The regular Oura Membership is available to all Oura Ring 3 or Oura Ring 4 members and requires a monthly fee.

If you have a Lifetime Membership, your Oura Membership remains free as long as you keep the same, active Oura account. Lifetime Memberships cannot be gifted, transferred, or sold to a different member.