The readers were counted as members of the clergy, so it was clear from other regulations that they cannot change their diocesan allegiance without the consent of their bishop. However, this canon states that it was enough to read once in a church to be treated somehow as a member of her clergy, and therefore barred from accepting ecclesiastical functions in other dioceses.
It shows that the laity were allowed to read publicly during liturgy; we know from other regulations concerning lectors that they were often established at a young age.
See [565] for the case which this canon probably refers to.