Hi Ansh,
If you are on ECC6 you can use FAGL_SWITCH_TO_OPEN_ITEM to Activate Open Item Management for Accounts Managed on Line Item Basis.
With this program even if you have the balance in GL account system will activate the check "Open Item Management" in gl master data, and also active the all the documents already posted in this GL, and make them as open on this date.
While executing this program you have to give document type and Account for transfer posting, and don't forget to select re-posting option as highlighted below.
No need to bring balance to zero, and don't block the gl also.
I hope this will work.
Regards
Javed