Yes copying scripts and plugins over is absolutely fine, just copy over your scripts and plugins folders from the old to the new.
What you don't want to do is copy over your userdata as it can cause problems.
2) In the video I recommend using the latest nightly builds from xbmc4which at the time was completely correct.
In other words don't come running to me with your bug reports - you've been warned I'm trying to do this as well. I just don't want to do this and then lose my files or ruin my Xbox, I've never done these kind of things.
I didn't even mod it I had to get someone else to do it for me.
Please also do join in the discussions over at our Forum.
See the development page for links to the source code and the SVN repository.
If you wish for XBMC to start at boot that is also covered in this section.