gripper change in pdf file..

Hi

Which is the easiest way to change the customer set pdf with gripper ...

For eg customer gave the imposed file with 50mm gripper from cut mark.. we need to change that into 60mm

we will place the pdf (20 forms) each manually (20 times) in third party software/ illustrator and move the gripper as required then output for plate...

we tried with preps but measuring from cutmark and alignment position takes more time any advise from experts pls...

regards
Kala