Starting with AIX 6.1 (TL4?), you can use the alt_disk_copy command to convert rootvg file systems from jfs to jfs2. The man page for the alt_disk_copy command states the following:
-T Indicates that you want to convert JFS file systems to JFS2 file systems during the process of recreating the rootvg volume group on target disks.
As an example, to clone a running systems rootvg to hdisk2, and convert the rootvg file systems from JFS to JFS2, you would run the following command:
# alt_disk_copy -T -d hdisk2
Good to know!