yep, the triple pattern does not allow you to use the first field as a real identifier. Just an intermediate identifier. That is, the <subject> will be replaced by the UID when the mutation is performed.
https://tour.dgraph.io/intro/5/
In theory yes, in practice you will have to edit your dataset. In all data, you must add a field for the new identifier.
Also you could check this pritice Get started with Dgraph and Get started with Dgraph