I would suggest one of two things in this situation :-
1. When you change the AS400 – set the new machine to have the same address as the old one, so you won’t need to change any external connectivity settings.
2. Use an automated scripting tool to change the settings on the PC’s. There are a lot of these tools around – some of them are even free. I have used one of these tools for something similar in the past – I will check which one I used and post it back here.