I did this - downloaded the demo and checked it worked for a day or two, then paid and got a key. I inserted it into the demo, activated it , then downloaded all default scenery in 2 long sessions. It works very well except for refusing to download one tile (+50+000 to +50 +009) so I wonder -
Should I have deleted the demo and downloaded the full thing from scratch ?
Can I do that now but keep a backup of the existing scenery and point it to that rather than re-uploading all of it? - this is the most important question.
How big is the base program minus scenery?
Should I have just renamed "X=Plane 10 Demo" and remove the word "Demo"
(There was nothing to stop me expanding the demo and I thought it would rename into the full prog after. Not knowing how big the base prog is made me try to avoid re-downloading all that anyway. I tried the rename - made no difference. One thing did make a difference to all problems except getting the missing scenery - I admit I didn't put X-Plane in the desktop but once I read how it needs to be there I moved it (I have avoided putting things there - used to be bad practice to just bung things there and clutter it up).
I expect many other people used the demo - buy key - expand with download route so this may be useful to a wider audience. Despite my problems this is still an outstanding sim - better than any other I have seen in 35 years of them!