We have Splunk 6.6.3 on a Windows 2008 server and need to migrate to a windows 2016 server. At the same time we are upgrading Splunk to version 7.2.5 What would be the best approach to this migration?
Check out What's the order of operations for upgrading Splunk Enterprise? for a visual walkthrough of the steps and links to the documentation you must follow to do this right.
I would highly discourage making two changes at once. If you run into a problem how will you know what caused it if making both changes? Check if Splunk 7.2.5 is supported on Windows 2008. If not, then you should upgrade the OS, wait a while to make sure it's stable, then upgrade Splunk.