When
The renewal application can be filed from 60 calendar days before your card expires up to 90 days after, with no penalty just for being late. Applying early gives you room for the fingerprint appointment, which is the real bottleneck.
The real order
First the renewal application; then the decision; only then the fingerprint appointment for the new card. Booking the appointment earlier is no use.
An expired card isn’t being undocumented
If you applied on time, your status remains valid while it is decided. The application receipt proves it: carry it with your expired card.
What changes at the appointment
Same appointment, same overloaded calendar. You bring the renewal decision and your previous card. The fee is paid again (19,30 €). Your NIE never changes; the card number and expiry date do.
General information, not legal advice. For questions about your own case, check the decision you were sent or ask a professional.