I believe the distinction between “flexible” and “fully flexible” is the ability to refund to the original form of payment.

Easiest online approach is probably to choose main cabin, then a prompt usually will come up on the next page to upgrade to “refundable” for a reasonable amount (I’ve been seeing $50 on one-way fares).

Nowadays there is no need to buy a full “Y” ticket as most main cabin domestic fares have a refundable option for a moderate up charge.