Before you switch
Check whether your current plan still has a contract; early termination may be charged.
Make sure the number is registered under your name, as the new provider will verify your identity (often via Singpass).
The usual process
Sign up with the new provider and choose to port in your existing number. The new provider handles the request with your old one.
You keep using your old SIM until the switch completes, then insert or activate the new SIM/eSIM. Expect a short gap in service during the cut-over.
After porting
Your old plan is normally terminated automatically on porting. Check your final bill and any add-ons you no longer need.
Some providers run port-in promotions — check the terms and the regular price after the promo.