Our NSM support contract calls for a CE onsite to do all upgrades of that
nature.
We went directly to 4.2.20 and AIX 5.1 with EC 17.

Aside from that, immediately after the upgrade, apply OS patch 5100-02
immediately.
It's a big one, 600+ MB.  From the looks of it, it affects almost every
fileset in the OS.
It fixes some duplicate IP address errors you'll see in errpt.

After than, double check your NIC settings - if you do not do
auto-negotiation on the link, you'll need to reset it.

If I had it to do over again, I think I would stay with something earlier
than 4.2.2 - there are some serious locking issues that some corruption in
our DB has made almost unbearable for us.

YMMV, of course.