Server migration, 2nd server as a backup

Hello,

planning to update production server (lets call it S1), need to add 8 disk tray, new HDD and reinstall all from 0.
So I have a backup server (lets call it S2). All 2 servers are not virtual but physical. I have one fixed IP address.

So my main question is how can I create an working S2 server and S1 can turn off and upgrade/reinstall?

+ S2 contains S1 webpage data. Have done it via CWP to CWP Migrate. So data is set. 
- S2 needs to have the same Hostname as S1? 

S1 now up and running, S2 is off.

Any info to understand how it needs to be done I would really appreciate.