1) If a user was part a plan that is now expired but I no longer want to offer it as an option.

Ensure you've plans available to upgrade into.

2) If is disable a plan where I have active members, will they still be able to login?

By disabled do you mean unpublished? The plan will no longer provide access at that point.

3) If the credit card subscription optin is being used (i.e. Stripe or PayPal) and a plan is altered or disabled, does it affect the payment schedule? In other words if a plan is no longer available will the auto-renewal still work?

Yes, recurring billing will still bill them. You need to cancel recurring payments. Ideally you shouldn't unpublish plans with active subscribers. Typically you should configure them to not allow new registrations, not allow renewals, and not allow upgrades that way those actively subscribed still function for their remaining time. You should then cancel the recurring billing manually, which in Stripe and PayPal you can set to cancel at end of their current billing cycle from within their respective dashboards for managing subscriptions.

If the only change is enabling family plans feature you shouldn't need a new plan at all. Simply enable that for the existing plan and should work as is. At any rate yes if there are no recurring payments you can simply move them to the new plan using either CBSubs > Import or manually moving them by editing them in CB > User Management 1 by 1.