Bechira - Choice, or Free Will
Q: Is there free-will in the Olam Haba?
In summation: Adam's natural energies were in line with the good but he had within him the potential for an emotional attraction which would be powerful enough to overcome his reason. Thus Adam can be summarized as a natural state of good with possibility of sin( ie., if external stimulus occurs which attracts sufficient energy to create a conflict between reason and emotion) Olam Habah is analogous to Gan Eden only re. the first idea. However it is different re. the 2nd because the possibility of a conflict in which emotion will contain the potential to overpower reason will not exist.
Q: I assume that sin would then be impossible in Olam Haba. Will this change the system of Mitzvos? Will we still have Yom Kippur?
It would seem that since there will be no sin there will be no reward and punishment, hence there will be no need to repent and hence no Yom Kippur. Bear in mind that the key point is that the Ramban identifies Yemot HaMashiach with Olam Habah which means the perfected state of the soul--the ultimate reward. Thus everything which we posit about Olam Habah. ie. "The world that is koolo Aruch, the world that is koolo tov etc means that it will be qualitatively different than the world we are in now. The Ramban accepts all the statements about Olam Habah which indicate that it is a state of absolute good with no evil. The only difference is that he maintains it will take place in the physical universe and man will have a physical form. However, he will be a perfected being , not subject to conflict, and incapable of sin. As such, there will be no need for Torah and mitzvot which were given to man in his imperfected state as a means of perfecting him and bringing him to his perfected state. Once in that state there is no longer any need for the system which brought him to perfection. (Once you are on the moon do you need the space ship which got you there? ie. if you have no intention of returning to Earth.)