The safest option is to back up before doing anything! If the data on your old os is important to you be sure to copy it somewhere safe before starting.
If your data is safe you could swap out the board and start up the pc from your old HDD. There is a very good chance the pc will start up and work after installing the new drivers. If that goes well you could access your data and get what you need.
If that doesn't go well go with your second option and install on a fresh HDD and use the old one as a second drive.
The thing is that your profile on the old OS may not be accessible from a fresh install.
As for MS office, if you have a license for one pc only you can use it on your new pc (only) or you old pc (only) not both. As long as it is only being used by one pc there is no issue.