No. Joint Cash accounts are not able to be converted into an individual 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 here.
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 Personal Capital Dashboard (not Cash account), please see How do I Remove a Joint Owner From My Dashboard?.