When you first upgrade to Featured or Premium, when payment is confirmed your listing will be Featured or Premium as appropriate for a period of 12 months to the day.
If you renew with say three weeks to go, upon confirmation of payment it will add 12 months to the current expiry date, ie 12 months + 3 weeks.
This is to the day.
If your Featured or Premium entry has expired, the chances are it will have already been set back to a Free listing and you will need to upgrade again.
However, if it is still within a week or two, it may still be set at Featured or Premium as appropriate. If you renew then, upon confirmation of payment it will add 12 months from the date of payment, not the expired date.
If a Featured or Premium entry does expire your images, any feedback and any stored enquiries will be kept, they will not be deleted - except any stored enquiries over three months old. But these additional services will not be accessible until you have renewed.