I was cleaning out my mailbox and noticed you didn’t receive a response. On the off chance you haven’t performed your upgrade yet, the easiest way would be to upgrade to V5R1 first and then you have a simple upgrade to V5R3. Of course, that depends on the hardware currently in place. Since you can’t order the software from IBM anymore, you might be able to get it from your IBM business partner (assuming you ordered via a business partner). Also, you might look at V5R2 but that has issues, too. Certainly, you could try to do a piece together upgrade, but I wouldn’t advise it. You might try a save of the user profiles and restore to the new system, but I don’t know if it would work from that far back.
If you can’t find a copy of V5R1 let me know. I may still have one buried in the storeroom.