Yes and no.

1) Clone disk using smit. (will need extra disk, or stop mirroring rootvg)

2) Boot from CD, choose migrate option using disk you just cloned.

Advantage is that if the migrated system is not working as expected, desired, you change bootlist to original (i.e. 5.1 root disk) and reboot and make changes as needed, and restart (clone, migrate). Downtime should be minimal.

Note: the actual migration process requires a boot from CDROM, and a migration. This largely determines your downtime.

Note: There is an option to do this migration 'live' using an NIM server. My systems have never been 'standard' enough for this to work, so you can try it if you have a NIM server (already at 5.3 btw) - otherwise it is a 'manual' process.