DOIFD will continue to work as normal. However you will no longer be able to receive updates including security updates. Future changes in Wordpress may also cause DOIFD to not function correctly as well.
Approximately a month prior to the license expiring you will receive reminder emails letting you know that the license will expire and that offer a 30% discount on renewal. This offer expires the day after you license expires. If you then decide to renew, you will have to pay the full price to continue to receive updates