This is the normal process. The current telco has to terminate your line before the new telco can port in your number. There will be a period of several hours where you are stuck in between.
I applied to port in, to Eight this afternoon. I just received an email (an hour back) from Simba that my line with them has been terminated. My eight is still showing the temporary number. Now waiting for eight to port in my line to their network. I am slightly worried as tomorrow is a public holiday. I hope I wont be stuck in limbo. Lucky thing is that these days hardly anyone calls me. The only gap is when I need to receive OTP number. Let's hope the port in is successful.
Update: Good news. My number was ported over to Eight at 1am. Huat Arh!