You can transfer the SSY account from Post Office to Bank. Total process is offline though hasslefree. Once I did the same with my PPF account too. You may follow below steps:
Wish to transfer your child’s Sukanya Samriddhi Yojana Account to ICICI Bank? It’s Simple – Submit your SSY transfer request to existing bank/PO, fill up a new account opening form with a set of KYC documents!
www.icicibank.com
Though, this is a link from ICICI Bank but it is same in every banks where you are comfortable enough & having good branch banking experience.
My suggestion to avoid PO, I experienced many uncertainties over matured amount withdrawal or proper transaction reliability. They don't have any transparency too & most of the POs are having cash shortages. If you ask any Cheque or DD they may keep you waiting for infinite times. People thought it's government backed, hence fully trusted but if a financial institution can't provide enough clarity over transactions then how can it be guaranteed! Moreover, you may many other hassles if you need any help from them.