Repeat: "and you have keys."
See WRKLICINF & DSPLICKEY. The keys on your current box will not work on
the new box.
You didn't say what version of the OS you are currently running.
But why stop at V5R4? Those newer machines can go all the way to 7.1.
Now, you'll have to make sure that your applications can run on that. Read
all three of the Memo to Users for 5.4, 6.1 and 7.1. Especially ANZOBJCVN
issues (but not solely that).
5.4 hits end of support in September. Since you may end up having to pick
up maintenance to get valid keys you may as well go the distance and get
your money's worth.
Another option to consider is replacing your tape drive. Compatible with
your current hardware may help your migration but I think it's time to at
least check out LTO. LTO holds MUCH more and is MUCH faster. You may
have to have your BP (or someone else) dupe your tapes to get through the
migration but a LTO drive will drastically reduce your future downtimes
due to saves. People are upgrading to LTO4 & 5 so LTO3's should be really
dropping in price on the used market. I think Larry Bolhuis can do just
about any tape duping. IDK what area of the country you are in. Sometimes
a talk with your local IBM CE can help you out with that. Gone are the
days when we had 5 different tape drives on one system. I used to do this
for people for logo'd shirts from their company and stuff. IDK what it
would take to hook up a LTO to your current system.