How complicated is it to reposition cheek implants that have been in place for over 20 years?

Replacing or repositioning your cheek implants that are asymmetrical should be done by an experienced facial surgeon. This will give the best results. I would need to see photos to give a more detailed answer.

Repositioning of cheek implants can be performed if they are not in the right position. They can also be replaced if necessary with  a different size or shape. In our practice, we performed Silastic cheek implants manufactured by Implantech. The implants can be removed,  exchanged or replaced under a brief general anesthetic as an outpatient surgical procedure. For many examples, please see  the link below

With such old style cheek implants, a better correction would be to replace them newer style implants that have lower profiles and blend in more naturally with the surrounding bone. While you can reposition what you have, it is not assured that their overall visibility will lessen appreciably as you have aged and probably have lost some facial fat.. But if only one side is the issue, then repositioning alone my suffice. It is not hard to reposition one of the implants I would just suggest that ti be screwed into place to avoid shifting back from where it is now.