My parents' visa is expiring on Apr end next year. They are planning to come on Dec and stay 6 months, so I am checking if anyone has experience renewing visitor visa before its expired (from Nepal). They have come here twice before.

I dont know about before its expired. But if your parents visa is expired. And if they have travelled US they just have to submit the ds form and dont even have to go for interview. I would say dont create a hassle renewing before. Ask them to come before their visa expires and leave. And let them apply after sometime like mth or so in nepal itself.