When you cancel an upload, you can continue it later by clicking the "Upload pictures" button again.
The problem with doing a modeless upload and allowing the user to change things around during the upload is that it introduces many possibilities of breaking things, for example if the user removes a picture as it's being uploaded, or changes the order, or changes a caption, etc.
And the benefit of being able to change the list of pictures to be uploaded is dubious. In a future version, I might allow making changes to other albums, but probably not to the currently uploading album.
In the meantime you can acheive the same thing by opening another GR instance (but be extremely careful not to upload to the same album from multiple sessions, Gallery doesn't handle locking very well, and you can end up messing your album up).