If I subscribe to Photoshop from the Creative Cloud will my present CS6, bought in the normal way, still be available if for any reason I decide not to proceed further with the expensive Cloud version ?
It really is worth reading through the FAQ http://www.adobe.com/products/creativecloud/faq.html
Yes they can be both be install on you machine and both will work. However CS6 will only able to open some of the PSD files created with Photoshop CC and as Photoshop CC continues to have additional features and options added more and more PSD files will not be useable with CS6. You have been locked in. If you stop paying Adobe your Photoshop CC will stop functioning and you lose the ability to use you assets till you start paying Adobe again so you can. You agree to this when you rent Photoshop CC and also agree Adobe has the right to stop service anytime and you will not hold Adobe responsible for any loss they have the rights to do so at their sole desecration.
How about making an action that flattens all your PSD files and then save them as tiff file. Winodws OS can candle tiff files. You can open the in Windows photodisplay and see thumb nails. Windows can display PSD thumb nails, I fyou have certain lyaer types starting in CS3 in PSd format Windows can't even read them and they can not be backup in Windows.
The problem your work assets are often PSD files that are templates which get populated with other assets. You stop paying rent Photoshop stops working and you have no way to process you work for now you can only rent you can not buy a working program.
If a file is written by CS3 to the windows file system. Windows can read and copy it and back it up. Windows may not know how to process the data in the file but windows can surly read and write the file