My husband is a CPA-expert witness (on real estate, financial, and accounting matters) and it is not uncommon to be making modifications to reports at the last minute. Attorneys are notorious for focusing on what on what is immediately critical and getting to other things as they become critical, so make their reviews of reports close to trial and there is often some back and forth between the expert witness and the attorneys over details as you approach trial.
Witnesses who have integrity stick with what their expertise (training, experience, judgment based on years in the field) tells them, but there are SOME witnesses whose integrity is quite a bit more elastic and they are heavily motivated to shape their opinions with what the attorneys (and clients) want to be heard/read. A courtroom is an adversarial environment and this "elasticity" tends to be revealed when the expert witness faces the opponent. Over time, reputations are formed and attorneys on both sides (as well as judges) gain an understanding of the integrity of the person sitting in the witness chair.