I have moved to logstash (combined with elastic search and Kibana) from Awsats. However I need to import/parse a few old IIS log files using logstash. The files has been moved to logstash server. However I was not able to find a command to parse the existing log files.
Here's a couple of links explaining what you'll want to be doing. Take a look through the rest of the documentation on the official site if you have any more issues.
10 minute tutorial. Part 5 shows what you need to be doing.
Parsing old logfiles with correct timestamps. Might be useful as you get into the swing of things.