I am not sure I understand your question, but you can certainly back up your system first using the Windows backup in the RC. You can then do a custom install to the Professional version. Your data will be put in folders on the new install called Windows.old and two hidden folders. You can recover it from there or the external drive if you want.
There is also something called Easy File Transfer which I did not use, but you might be able to.
You might look around the internet a bit to see if you can find any more information about your type of situation.
Also go through some of the threads and tutorials here to see what kind of problems some folks are having and how they might be resolved before you do the install, just remember more accurate information usually comes later down the thread so check the dates.