After the fails I experienced last time, I went back to the first store to switch my pay & go price plan and transfer my credits.

I explained two times what I wanted before actually reached the counter and being asked again. I asked the staff if there was the only store can do this transfer and they all gave me a no.
"It is the computer system do the transfer, every store can do this."

After explaining I was neither from other networks nor wanted to switch to a monthly contract, they finally understood what I wanted and I still got turned down.
"We do this transfer once grouped a number of application, about once a month, but I still don't know about when is the next time on this month so I can't apply it for you now. You have to come again later."
"It's once a month and normally at the middle on the month, I know this month is done on about 15th or 16th. Can I apply it in advance? I know someone applied it at late September and got transfered at mid October."
"Middle of the month? I haven't been told that. Not even on our briefing book. I can't apply for you now because I have to submit the application form as soon as it's filled but I can't write the transferring date at the moment. It's easy a job, just done by the computer system, don't even need to switch the sim."

*Sigh* If I don't apply in advance, how can you "group" the application form? What if I come next time and you say this month is done again? I know it's easy and that's why I was there.