If your property was financed in whole or in part using CPF funds, selling your property after you have turned 55 has implications with regards to the amount of refund you need to make into your own CPF account.
Before turning 55, the amount you will need to refund is the cumulative total of all principal sums (P) in your CPF account used by you to pay for the downpayment and monthly instalments for your mortgage loan, plus the accrued interest (I) arising from these sums. This P+I repayment is supposed to put you back into the same position as if you did not use the money for housing purposes in the first place i.e. sitting in your Ordinary Account and earning interest. The refunded amount, together with any existing Ordinary Account balance can then be used for your next property purchase.
After turning 55, the same refund of P+I is required, and additionally, if you had pledged the property to make up your Full Retirement Sum (FRS) upon turning 55, this amount will need to be refunded together with the P+I. This way, your Retirement Account is topped up to your FRS. Any balance will then be paid to you in cash or you can choose to keep it in your CPF Account. You can reuse the Ordinary Account balance to purchase your next property, subject to the applicable housing rules and limits.