Can a retainer be used to reposition my teeth once they have shifted slightly?

Yes: A spring retainer can be made from a model where the front teeth can be repositioned to a corrected position, however, anterior teeth may need to be slenderized to gain space to fit the teeth back into position.

Perhaps, it depends : If your retainer no longer fits then you. Need to see your orthodontist or your general dentist. You are a good candidate for invisalign. This should only take a few aligners to fix. If the retainer fits completely but is just tight then try wearing it as much as you can and this can move the teeth slightly.