No penalties as such, but bank would be answer that in a better manner. It's just a self declaration.
Transfer from NRO to NRE would need Form 15CB CA and Form 15CB would be issued by CA after checking that the money you are sending to NRE, tax has been properly paid on same.
DTAA doesn't provide any relief from the money deposited in NRO account.
There is a requirement of 15CB as mentioned above.
NRO account taxation and interest is more or less similar to normal saving account.
Hope you find the information helpful if you do please rate it 5 and provide your valuable feedback for my improvement.
Thank you