When is the latest I can switch to private kkh or Thomson

Hi, first time mummy here at week 8. Currently under subsided at NUH. I do have plans to switch to private in future for a better comfort and lesser waiting time if our wallet allow us to. May I know is there a big difference coming from nuh subsided to Thomson private hospital? Is it worth the money difference? And when is the latest I can swap to private. Planning to take advantage of the subsided for abnormalities scans for now. Please advise me ladies