Hi,
You can see if your key will work by trying to download Office from here:
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You will have to re-install Office when reformat your machine. Do you need to uninstall it first? No not really just go ahead with the reformat.
One thing you could try is downloading this 60 day trial and see if it can be upgraded to a normal version using your keys? I'm not sure if this is even possible but maybe it might work?
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Another thing you could do is if you end up losing the ability to use Office there is a freeware equivalent called 'Open Office' which has pretty much everything Office has and is compatible:
https://www.openoffice.org/