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.
