I changed ff-programs segment by segment more than 100 times in the last 5 years.

About 60% were changed correctly right away.
About 30% of the time the first agent either refused to do it, couldn't do it (lacking know-how), or did it wrong. I learned to recheck (or let it redo) by a second agent (at the lounge, at a transfer desk or at the gate).
About 10% didn't work out: 5% didn't get changed , 5% got credit in both ff-programs It never happened that a change did rewrite old segments, but often (in a majority of 'cases') the change did change all future segments (with the same ticket).