Moving from a flat to a condominium is two transactions, and the order you do them in changes what you pay by tens of thousands of dollars.
Sell first, then buy
You own nothing when you buy, so there is no , the loan is the full 75%, and you know exactly what you have to spend. The cost is practical: you may need somewhere to live between completing the sale and collecting the new keys.
Buy first, then sell
You can move straight across, but at the moment you buy you own two homes, so a citizen pays 20% ABSD on the new one. Sell the flat within six months of completing the purchase and you can apply to have it refunded. On a $1.8 million home that is $360,000 to find and then wait for.
Keeping the flat
Holding on to the flat and renting it out makes the private home a second property for good: 20% ABSD with no refund, a loan capped at 45% of the price, and at least a quarter of it in cash. The rent has to do a great deal of work to justify that.
Coming back the other way
Downgrading is not symmetrical. A former private-property owner must sell the private home and then wait 15 months before buying a resale flat without a subsidy. The wait is waived for buyers aged 55 or over moving to a 4-room flat or smaller.
The upgrade tool runs both halves: what the flat leaves you after the loan, the agent and the CPF refund, and what that reaches on the private side.
Source: HDB: buying a resale flat after owning private property. ABSD remission has conditions and a deadline; confirm them with IRAS before relying on it.