Clear Opt out selection for a number

We used to use HV-SMS to text prospective employees, but now we want to use it to text ONLY employees. However, a number of people who have opted out before now want to get these sms messages. How to I reinstate a number that I want to text? That is, clear their opt out?

If that can't be done, can I change the number that I'm sending from or will the opt out still apply?