Check your remaining amount payable
[Remaining Loan Payments] + [Interest from Bank Loan or CPF Accrued Interest]
Determine a Profitable Sale or Negative Sale
From the information gathered above, you will be fairly certain whether selling your HDB Flat and moving into an EC will be possible or not; including estimates of cash proceeds.
However, if you've already found yourself reading this, profitability is less of a concern. HDB to EC upgraders like yourself have come into money and are typically more concerned with a lifestyle upgrade instead of capital gain profits.