What portability actually means
Worldwide cover tells you where you can be treated. Portability tells you what happens to your terms when your life changes country. They are not the same feature, and only one of them is tested by a move.
Anyone who has already moved once knows the handover is where things go wrong. New job, new address, new plan, and somewhere in the middle a form nobody chased. The cover itself is rarely the problem. The join is.
So the question worth asking of any plan is not only what it pays for. It is what would still be true if you were living somewhere else in 18 months.
The history you have already disclosed
Keep the same policy through a move and your underwriting terms usually travel with it. What an insurer accepted at the outset generally stays accepted. Start again with a new insurer and you are underwritten on the health you have now, not the health you had when you first bought.
Waiting periods you have already served
Most plans count waiting periods from the date cover started, not from the date you arrived in the new country. A new policy usually restarts those clocks. Maternity is where that hurts most, because the wait is long and the deadline is not negotiable.
Where you are covered is a setting
Area of cover is usually something you change on the policy rather than a reason to change insurer. Move from Asia to Europe and often it is the region and the premium that move while the plan carries on. The United States is handled separately on most plans.
What does not carry across is anything tied to the country you are leaving. Local hospital networks, the settlement arrangements behind them, and any benefit written around the Singapore system all change when you do. And nothing carries at all if the policy lapses. A gap of even a few weeks is usually treated as a new application, which means fresh underwriting and waiting periods starting again.