It is ok to change / rename hostname for Oracle Application Server Infrastructure and Midtier. However documentation says it does not support Oracle AS with Metadata Repository.
Simply execute the following command to change hostname for Midtier.
% $ORACLE_HOME/chgip/scripts/chgiphost.sh -mid
The script is clever enough to pick up the host information. So you just need to accept the default prompt 4 times, that's it.