What are your thoughts? could wearing a retainer that doesn't quite fit my teeth move them to the wrong shape?

Yes: Wearing an ill-fitting retainer can allow the teeth to shift back or may push them out of position. This may cause you arch to be misshaped, the opposing teeth to move and throw your bite off. Go back to your orthodontist or dentist for an evaluation. They may be able to adjust the retainer to fit properly or take an impression for new appliance.

DONT DO IT!: If a retainer does not fit correctly do not force it on, consult your dentist to either have a new one made to retain what you have now or consult an orthodontist to have your teeth corrected and proper fitting appliance made.

Yes!: If the retainers are ill-fitting then you've changed and they shouldn't be used. Seek the advise of your orthodontist/dentist to evaluate the retainers and you to help guide you to the best course of care for your present tooth alignment. Good Luck!