If you sell goods or services that will be delivered or used before and after the GST increase, there are certain rules you need to follow. This may include adjusting the GST percentage you charge on those goods or services and making sure to charge the new GST rate on them.
The IRAS has prepared a guide for GST-registered businesses on the GST rate change.
We have extracted the key information here for your reference.
Note that the above does not apply to reverse charge cases.