Physical shares held in a joint account can only be Dematerialized into a joint Demat account in the name of the joint shareholders. You will not be able to convert it to a single holder Demat account.
Note that the names on the joint Demat account must match the names on the share certificates to Dematerialize jointly held shares.

When the order of names on the share certificate differs from the order of names on the joint Demat account, you must submit the transposition form along with the relevant papers for de-materialization.