A plain look at what changed, and what it means from here.
Most people meet this once and never think about it again, which is exactly why it is worth reading up on.
The exceptions
Nobody is required to explain this to you, which is half the problem. The practical effect is easier to see in a single example than in a summary.
What to check first
Nobody is required to explain this to you, which is half the problem. There is a deadline involved, and missing it costs more than the difference.
None of this is urgent, but it is worth an hour before the next renewal.