Hi there! You are required to make:
1. Payment of S$100 administrative fee
2. Payment of road tax under the Normal Car Scheme
3. Top-up the unused upfront rebate (if applicable)
After conversion you must change the car registration number plates to reflect the car’s new scheme within 3 calendar days from the date of conversion.
Also, you have to submit a few documents which can be done at LTA Customer Service Centre.
You can find out more here: https://www.lta.gov.sg/content/ltaweb/en/roads-and-motoring/transport-options-for-motorists/revised-off-peak-car-and-opc-and-weekend-car.html