Data migration historical P&L

Dear Community,

Our Go Live is 1st of July and our customer requires the data migration of historical P&L for every posting period year to end (mainly for sales accounts with profitability segments - profit center, distribution channel, customer group etc).

Using Historical balances data migration object is not possible because we don`t have the characteristics needed.

Using GL Open items migration object - it is not possible because the posting date is taken from the Define Settings for Legacy Data Transfer.

Should I use Upload General Journal Entries App? Do you see any issue if I post documents with a posting date that is before the one set up in the Define Settings for Legacy Data Transfer?

Or should I use the GL Open items migration object and play with the migration key date?

I am afraid that the system will have a different behaviour in production and it will not allow me to use the scenarios described above?

Kind Regards,

Diana