There's no fee for changing your ticket after a schedule change, nor do you need to worry about fare buckets in your cabin class.

If you change the ticket due to the schedule change, the original fare rules "should" still apply when you later try to SDC, although it's possible that you can get DL to now give you an illegal routing due to the schedule change.

I'm not sure whether DL will give trouble about more than four segments in each direction.