Subsequent Activation of Open Item Management

We are on ECC 6.0 Version.

While creating a GL account during Go Live, open item management was not activated ( line item display is active ).

Now we need to convert the same account to open item management activated.

But the GL already contains few records which also need to be converted as open items.

Also New GL functionality is activated for the company code ( Document Spliting based on Business Area ).

How to convert the existing as well as new items to open items in the GL ?

Whether additing Development Pack 3 after 3 months of GO Live will affect any customisation ?