No. Joint Empower Personal Cash accounts are not able to be converted into an individual Empower Personal Cash account. This restriction would also apply to a Joint Cash account which was opened as Individual Cash account, and was later converted into a Joint account.
If you no longer desire to have a Joint Cash account with the other owner, you (or the other owner) will need to request that the Cash account be closed by following the steps outlined in this Link.
Please note: Either owner of the Joint Cash account can close the Cash account on which they are listed as a joint owner.
Closing a Joint Cash account does not remove the joint owner from your Dashboard. To remove a user from your Empower Dashboard (not Cash account), please see How do I Remove a Joint Owner From My Dashboard?.