Yes! You can upgrade your $5/mo for 2GB Plan to the $28.26/mo Base Plan.
However, you cannot downgrade from your $5/mo 2GB Plan to the $0 Flexi Plan.
Please note: Starting 1 May 2023, customers will see the 8% GST mandated by the Singapore government applied on specific products such as Base Plan and Unlimited Data Add-on amongst others. For the full list of products impacted by the GST hike, please refer here.