When moving from one position to another you must keep your fingers in contact with the string. To achieve this smoothly and efficiently, it is essential to relax your left hand and thumb. Try shifting with your thumb loosely hanging away from the back of the cello neck, to give you the feeling of freedom required.
As an eercise, try slow shifts from 1st position on any string right to the top of the cello. It’s not nearly as difficult as it seems. Go on, have a bit of fun. Do it when nobody can hear you, so that if it sounds like you are stragling a cat, you won’t feel self-conscious. Go up the string on a down bow and back down again on the up bow.
See the picture at Dr Downing Music to see the shift in note form.
This information is available at http://www.drdowningmusic.com/images/Strings/Cello/cello1.GIF if you want to see it small and not readily printable.