Hi Tomoyo
From your query, I understand that you actually are confused between Profit Center Accounting being Activated and Profit Center field to be auto-populated. Or maybe I am.
Profit Center field is auto-populated from FI Document. Therefore you need to ensure that while posting an FI Document, Profit Center is field (Through derivations or manual input) is maintained. For this reason, you do not need to have Classic Profit Center Accounting Activated.
If however you are using New GL and still need Classical Profit Center Accounting, then I would be interested to know why. The reason being it populates database GLP* in addition to FAGL* Tables and this need not be required unless there is a strong business case.
Regards
Mustafa Sakerwala