About a quarter of the 1.8 million users of TraceTogether have updated their app to the latest version, which requires them to re-register with their identification numbers.
These 450,000 app users had previously entered their mobile phone numbers, but the new requirement to supply more personal data is needed under stepped-up measures to combat Covid-19's spread.
A spokesman for the Government Technology Agency (GovTech) told The Straits Times that all versions of the app will still work in exchanging and logging Bluetooth signals with nearby devices.
In the event that someone is infected with the coronavirus, the Bluetooth data logs in the devices will quickly unravel who had been in close contact with the patient.
GovTech said that making app users re-register with their NRIC numbers, foreign identification numbers or passport numbers will allow close contacts to be identified more accurately and quickly.
For instance, people may have multiple mobile numbers registered in their names but their family members or workers may be the ones using the numbers.
Mr K.K. Lim, head of cyber security, privacy and data protection at law firm Eversheds Harry Elias, said: "So, for example, Mr A may own several mobile phones all registered under him but then he gave them to different workers. Tracing by just the mobile number is problematic."
GovTech urged users not to delete the old version of their TraceTogether app as it will result in a loss of previously collected data. Rather, they should update their existing app with the latest version.
The 1.8 million users who have downloaded TraceTogether represent about 25 per cent of Singapore's population - still short of the minimum of 75 per cent needed for TraceTogether to work well.
The updated TraceTogether app also carries a unique NRIC/FIN barcode that belongs to the user, enabling faster check-in at stores, buildings and locations with the national digital check-in system SafeEntry.
The Smart Nation and Digital Government Group (SNDGG) said that having a common data field such as identification numbers will allow greater inter-operability between TraceTogether and SafeEntry.
For instance, contact tracers can identify the close contacts of confirmed Covid-19 patients by their NRIC numbers and locate their home address, even if their phone numbers change or their phones are switched off.
SafeEntry and TraceTogether are currently two separate systems, said an SNDGG spokesman.
"SafeEntry collects check-in and check-out data that helps contact tracers establish clusters, and potentially find the source of infection, while TraceTogether helps to find links between close contacts," she said.
"Both are important for contact tracing and the speedy isolation of suspected cases."
The Ministry of Health (MOH) had said last week it does not use SafeEntry information to identify people who had gone to the same places visited by confirmed Covid-19 cases.
The data collected by SafeEntry is used by contact tracers when interviewing patients to help them remember where they have been and create a robust activity map.
MOH did not respond by press time on how an updated TraceTogether app will be better integrated with SafeEntry for contact tracing.
But Minister-in-charge of the Smart Nation Initiative Vivian Balakrishnan had said on Monday that the use of digital tools to trace the close contacts of an infected person will take less than one day.
Without digital tools, finding potentially infected individuals could take days as human contact tracers need to jolt the memory of Covid-19 patients to identify people who have been in close contact over the 14 days prior to the onset of symptoms, and make calls or go door to door to reach these people.
SafeEntry has been installed in over 110,000 locations including malls, supermarkets and workplaces.
A list of places of where confirmed community cases had visited for more than 30 minutes, along with the dates and times, is available online at gov.sg
People who have been to the same locations in the same time period should monitor their health closely for 14 days from the date of their visits, and seek medical attention if they develop symptoms.