My subscription to the tickets feature expired, can I renew my subscription and will the tickets that have been sent be valid and can I send tickets to those who didn't get their tickets yet?

Yes.

If you subscribe to the Custom Tickets features the tickets that have been distributed will work. You can also send the tickets to the attendees who have not yet received a ticket by clicking the Resend Ticket Notice button associated with their registration.

You can find your attendee that did not receive their ticket yet in two ways:

1. Search for their name or contact information. Navigate to Registrations > Search for their name. 
2. Look through the list of attendees for your event. Navigate to Events > and click the "Registrations" link under the event name: 
   
   OR  click the Attendees number under the Green people icon:
   OR people action link for the event: 

You will then see the list of attendees for that event.

Once you find  your attendee, you can click the "Resend Ticket Notice" button for that attendee: