You don't change the tags. Sheep are tagged at birth and that same tag follows them through life. That is the whole point of tagging - so an animal can be traced from birth to death.
You probably need to inform DEFRA that the flock and holding number now belong to you, but I don't know about that - who you inform varies through GB. You also need to mark the transfer in your movement record, and take over your grandfather's sheep records.