Set PR as unchangeable (ME52N)

Hi,

I need to set a PR as unchangeable when it has been referenced to PO.

Is it possible to do it by customizing?

For example,

We created 100000000 PR, then we created 4500000000 PO (with reference to 100000000 PR).

If We try to change any field in ME52N of 100000000 PR, the system must not allowed it.

Thanks,

Luis Ganoza