Hi,
How are you creating the PIRs ?
Is it manually or by LSMW or any other upload program ?
If it is through program, with all other mandatory data, use default Per Unit as 1000 in the file mapped in the program.
Else
You will have go for ABAP development as there is no standard setting to achieve this for the PIRs which are already created.
through program, You will have to delete the existing PB00 condition in "Condition Tab" and recreate with Per unit as 1000.