Who told you to Actually port out,
I suggested requesting a UPC(Unique Porting Code) as after that most operator's Retention Team will call you and offer dicsounts
and very prompt solution to your issue.
you can try to convert to Pre-paid and if local store denies, just come back, request UPC and wait for retention team to call you, once they call,
tell them, you are loyal customer but don't want postpaid, you would retain their only if they convert you to pre-paid.
I am sure you would be converted to pre-paid smoothly.