Moving all photos from one album to another

I recently uploaded a few hundred pics into an album, then decided I wanted to split the pics into sub-albums.

I created a sub album, but can't figure out a good way of mass-moving all the pics from the album into the sub-album.

I basically I have this:

gallery -> album1 -> sub-album1

And I want to move all the pics from album1 into sub-album1. Is there an easy way?