The German localisation package allows the users to choose which structure they want to use for their accounting: the SKR03 or the SKR04 structure. It is very important to align with the company's financial strategy and choose the one, which fits to it. 

A change between the 2 structures is only possible until the first accounting entry is posted. Once there is anything posted, the option to select between the 2, disappears and it will not be possible to change anymore. At the time of the first entry, Odoo will go with the structure, selected at that point, and associates the accounts of the selected structure to journals, taxes, contacts, products and product categories. 

Though these associations can be changed manually as per the company's needs, it is only advised to select from existing or additionally created accounts and no accounts which were used before, should be deleted.