What can be done about my upper anterior crowns that are suddenly shifting after 15 years?

If you like your crowns still and they are in good shape, then use Invisalign to get them back into alignment.

If you're teeth are shifting, it would be a good idea to consult with an orthodontist. But teeth don't just start to shift after 15 years without reason. The underlying reason would have to be addressed to prevent it from happening again.