Windows OS upgrading from 2008R2 to 2012 on SharePoint 2013 hosted servers

As far as I know, in place upgrade of windows server is not supported for SharePoint servers. It is kind of full rebuild of the SharePoint farm. Check this link.

• Make sure that the intended operating system is supported by the version of SharePoint that you are running.

   

• Detach the content databases from the SharePoint server.

• Uninstall SharePoint through Control Panel.

• Upgrade the operating system.

• Reinstall SharePoint.

• Attach the content databases.

If it is single server farm then above is only options. But multi server farms you can take one server out of farm rebuild it and join it to farm. This way you will have a mix os in farm for short period of time which is ok. This process will minimize the downtime.

Regarding the Possibility

In general scenarios, yes it's possible to upgrade the windows server from 2008 to 2012 in-place

But it's heavily recommended to do not do that on the production servers.

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Regarding the Effect

To be honest, I don't try it before for SharepPoint farm to can specify specifically what's the effect, but based on my knowledge it for sure will effect on the farm at least there is a long downtime that may be forever!!.

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In case you want to venture on test environment try to do the following:

• Take a full farm backup
• Take SharePoint content databases backup in case you want to use Database-Attach method.
• List all Applications service, Important configurations ....etc
• Follow the mentioned steps to upgrade windows at Performing an in-place upgrade of Server 2008 R2 to Server 2012