what is the availability using new mapping set and override the account enter in po

our case is we change the account mapping set and account roll For purchasing charge account, Currently we have many po using the old mapping set but the create accounting still not generate.

So what is the function require to generate account for this pos using the NEW mapping?

"what is the availability using new mapping set and override the account enter in po while we do not create account for Receipt and Costing"