You would completely redo the computer with a clean install. You would then add all of your programs again. This is something that many of us have done many, many times. I suppose (I don't know) you could update by installing over the other OS, but I would have to look at that to see if you could do that.
Either way, you are MUCH better off starting clean. You would not do a backup of the computer as you will be installing WIN 7 Pro fresh. Many people reformat and install the same OS over just to have a clean running computer as after a while you get registry errors and corrupt files, etc.
It is always a good idea to write down a plan of how you want to reinstall. Again, not using this computer to work on makes it easier. You just install WIN 7 Pro 64-bit and install AC and you are good to go.
