Re: Upgrade of NetWorker Server
Of course Hrvoje is correct.
On the other hand I do not see an issue with the OS in general - You will be able to install NW 7.6.5 on W2012R2 - i just verified that. Although I did not convert any databases, I do not see an issue here if this is just an interim solution.
If you have time you can of course test this with a standalone server first - you do not need extra backup hardware.