People will come and people will go. They'll change jobs and roles. Your Cards and Skills need to go with the flow!
If a Mentor leaves your Team for any reason, you can easily replace him/her in your Cards and Skills without closing them for maintenance. As an Author, you'll now see the normal Edit button even when the Card or Skill is still Open. But when you click it, you'll only be able to change Mentors - any other change to the Card or Skill requires you to Close it first, so Pract.us can keep track of the changes.
If a Learner is enrolled in a Card, and Mentors change for any reason, then the Learner's Card will update automatically, but only for those Skills that have not yet been signed-off. The Sign-off is a permanent record of accomplishment, and Pract.us keeps that record intact no matter what happens later. Same is true for Completed Cards - what's done is done and won't be affected by later changes.