Under FEMA and RBI regulations, a person who becomes a Non-Resident Indian is required to convert their existing resident savings account into an NRO account, or close it and open an NRO account, once their residential status changes. It is not legally permissible under FEMA to continue operating an ordinary resident savings account once you qualify as an NRI, even if the bank never flags it.
When 'becoming an NRI' actually triggers
The trigger isn't a visa category or a round-trip ticket -- it's residential status under FEMA, generally understood as residing outside India for more than 182 days in a financial year, or having a clear intention to reside abroad for an uncertain period (for example, taking up long-term overseas employment). The RBI rule applies from that point regardless of whether the person has told their Indian bank yet.
What's actually at stake
This isn't a purely theoretical compliance point. Continuing to run a resident account as an NRI can attract FEMA penalties reported to run as high as three times the amount involved, and any linked fixed or recurring deposits need to be converted along with the savings account, not left as-is. Most CA and legal guidance recommends starting the conversion within roughly three months of qualifying as an NRI -- there's no statutory grace period, so the safer approach is to treat the day residency status changes as the day to start the paperwork, not the day someone happens to remember.