The policy on how a nonrefundable ticket can be reused varies widely by airline. For example:

With UA,a ticket is good forever, the $75 change fee can be deducted from the ticket value, and if there is a residual they issue you a voucher that is good for a year.

But with Alaska, a ticket is good for one year, you pay the $50 change fee in cash (even if the new ticket is
less expensive), and if there is a residual they won't issue a voucher - you lose it.

So be VERY sure you know the rules before planning to reuse a nonrefundable ticket!!!