For most people changing positions is extremely stressful. They are filled with fear of the unknown, doubts from family and pressures from peers at their old position. Anything could trigger a reversal of their decision. They need to keep reaffirming that they have made the right decision and you need to let them know that you are right there with them. Call them the day before and go over all the reasons again and make sure they are comfortable with the decision. If you sense any doubt it may be red flag or a warning of something to be watchful for over the next few months.