It copies the files over to the gallery. If it just moved them then that would probably create a headache with the files that you don't want moved and such. I'm pretty sure vB
has some options where you would easily be able to remove the attachments afterwards as well.
It shouldn't affect your posts/threads in any way.
The script has some redirects in place, so even if it does take it a little while to move all of the files you still should be able to just leave the window open while the script is doing it's thing.