Hi
First thing if you say that you have to use same PA means you will have to use same MPOS as assignment can't be changed.
Now you are reducing which means you need to deactivate PA & MPOS. Make changes in the MPOS. This might lead to inconsistencies. I had same requirement in one of the projects and we created a new PA & MPOS which was much more easy to handle.
Check the warning mentioned for the deactivation in the below link.
Working with Master Planning Object Structures - Planning Area Administration - SAP Library
Rest you need to convenience your client.
Regards
Rahul