I tried to change it in smitty under:
Change / Show Characteristics of a Journaled File System

and I get this error:

COMMAND STATUS

Command: failed stdout: yes stderr: no

Before command completion, additional instructions may appear below.

chfs: 0506-932 Can not extend a JFS file system with nbpi = 4096
and frag = 4096 past 134217728 (512 byte blocks).