You could also create a lookup table that maps ip addresses (or old hostnames) to current hostnames. Set it as an automatic lookup and you will always have a field that represents the current hostname. You will only have to maintain a text file (CSV) of the mapping - and you could automate the update of the CSV file.
http://docs.splunk.com/Documentation/Splunk/latest/Knowledge/Aboutlookupsandfieldactions
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