So... I believe my moving is succeed.
For those who are interested:
if you have a gallery with too many subcategories make them top categories and it will be easier for you to find their parents in new PP gallery.
I had some troubles with that as some subcategories changed their parents and if your gallery is as large as mine it's difficult to find a new place.
I used comments search to find some subcategories.
And now everything seems ok, though not all the photos moved, may be some of them were corrupt.