I just want to know if is there some impact changing the hostname of a linux machine. (Only the hostname not the instance)
If you are using a Deployment Server, you may have to update the whitelist (or possibly blacklist) entries in any serverclass entries of which the host is a member. It is possible that it might be referenced directly in a search or lookup file.
If you are using the machine as forwarder than you need to update the whitelist in serverclass.conf.