You would have been issued with a Re-Entry Permit (REP) when you received your permanent residency.
A valid REP is necessary whenever a permanent resident (PR) travels out of Singapore. The REP allows you to retain your permanent residence status while away from Singapore.
If you leave Singapore or remain overseas without a valid REP, you will lose your permanent residence status. You will be assessed for entry into Singapore as a foreign visitor, subject to prevailing entry requirements.
If you change a new passport of the same nationality, you will need to update your new passport details in your REP promptly using the transfer of REP function. Please note that the checkpoint may only accord you clearance as a PR if you have a valid REP with the updated passport details at the point of entry. If your REP is not in order, please inform the immigration officer of your PR status.