These are the steps to change host name in the app also, not a nice one:
1) Make a full backup of your XI 3.1 system (both database and file repository)
2) Change the hostname
3) Create a new SIA with all defaults servers using the CCM. Make sure that you instruct the CMS to use the same database connection as before.
4) Run the "Update objects" command in the CCM on the new SIA
5) Stop the old SIA and change the ports for the new SIA and the CMS to the default values (if required)
6) Remove the old SIA in the CCM
7) Go into to the CMC and try to delete any remaining servers from the old SIA.
Please check with support before performing the steps.