Patty:
Have you moved out of the area of the old parish, and into the area of the new?
If you have moved, simply register in your new parish, and ask if they inform your old parish. If they do, fine. If they don’t, simply send a quick note: “Due to a change of address, I have registered with St. Polycarp’s. Please take me off your rolls.”
The biggest and best reason (from a parish’s point of view) for the note of change is that many dioceses “tax” their parishes based on the number of enrolled parishioners. This diocesan assessment is obviously necessary for the diocese to operate. But the old parish will want to remove you from the rolls because of this.
If you are not within the geographic boundaries of the new parish, or if it is a “personal” parish (erected to serve the needs of, say, a particular ethic group) and you are not a member of the group it is intended for, the pastor may not accept your registration. This may be a diocesan rule, or it may be a parish rule. But you may still attend Mass in any Catholic church or oratory.
Regards,
Joe