This issue has actually never come up very often in my career, which often surprises me, but yes—within a particular run, monologues can become very similar from night to night. They are still performed extemporaneously—there's no script, and they often begin changing again once something shifts, be it a run beginning or ending, time passing, events from the world impinging, or unidentified forces. There are times when the performances are almost crystalline from night to night, and the music of particular beats are nearly the same. Other times there will be seismic shifts and the show will change in surprisingly large ways, losing parts of scenes, gaining new ones, and the wordings shifting to accommodate these changes.
Extemporaneous isn't improvisational, and it isn't scripted, but the shows do indeed change all the time...but they also change less as they grow more mature, just as we all tend to change less as we discover who we are as we grow up.
It's a lengthy topic, but I'll leave it here tonight to simply say no, the show does indeed change all the time. For example, the final scene has not been performed the same way more than once since Tuesday, as the show is still changing in the new circumstances after Steve's death.
