Billing date for exising invoices

Hello experts,

we have one urgent issue to be resolved. There are around 1500 Intercompany invoices which have been created after month close (on 29 july) instead of correct time. and hence there is a huge differnece in reconcillation. any possible solution to change the billing date in exising documents ? or may be adviced to do the necessary corrections. Bit urgent and lost.

thanks

Jignesh