Reinstalling Windows 2012 R2 on T320 questions

Something has happened to my server OS and it now wants me to activate it again except it won't accept the key code from my Dell T320 server which is the original key that I used to activate it.  It also won't start the Software Protection Service and subsequently won't accept any updates.  After two days on the phone with Dell support and also Microsoft, Dell says that i have to reinstall the OS.  Not happy about this as my client can't be down.  Anyways, I want to know if I can do this remotely using my iDrac that I have setup for the server?  Will I loose connection when it reinstalls?  Also, can I do an upgrade install to just refresh the OS?  Can I do a Factory Restore using the Dell Restore Partition?  Will this have all the drivers that I need, etc.?  Also, I have a separate partition setup for all the data files and I want to know if this partition will be left untouched when I do a Factory Restore?

I have never done this before and the system original came loaded with the OS so I am nervous to not screw this up.  Is there a way to somehow backup and restore the AD users?