Hi Neehal,
You assign a planning area to a MPOS, which in turn is assigned characteristics and aggregates. Since your planning area is based on an existing MPOS, you won't be able to change the MPOS name in planning area created.
If you want your planning area to be based on new MPOS, the you have to deinitialize the planning area, delete the time series data and then have it based on new MPOS.
Regards,
Alok