Redjericho,
After AT&T started the "Unlimited Data" plan they realized that they were not making enough money at it, so they decided to end it. However, they had a considerable number of people signed up for it and they couldn't just drop them. So they said that if you were on the plan already you could keep it. but if for any reason you left the plan you couldn't come back or restart it.
To be "Grandfathered" into any program means that you were in it before, say, certain changes were made, or maybe the plan or program was ended and you can continue to use the old rules or keep the plan. You are "Grandfathered" into it.
After AT&T started the "Unlimited Data" plan they realized that they were not making enough money at it, so they decided to end it. However, they had a considerable number of people signed up for it and they couldn't just drop them. So they said that if you were on the plan already you could keep it. but if for any reason you left the plan you couldn't come back or restart it.
To be "Grandfathered" into any program means that you were in it before, say, certain changes were made, or maybe the plan or program was ended and you can continue to use the old rules or keep the plan. You are "Grandfathered" into it.