AARON - I remember reading somewhere that it will be more difficult for people to change and accept the gospel in spirit prison or without a body. Do you know the source?
JOEL - It won't be more difficult in the spirit world to accept the gospel for those who would have accepted it on earth if they had the opportunity. God is fair and just and would not give an unfair advantage or disadvantage to anyone when it comes to their eternal life. On this Joseph Fielding McConkie said:
"If God is just, then all of his children must have an equal opportunity to accept or reject the gospel before the Day of Judgment. As Latter-day Saints, we know that those who did not have the opportunity to accept the gospel in this life will have it in the spirit world before they are called forth from the grave. These, Peter said, will then "be judged according to men in the flesh" (1 Pet. 4:6). This statement means that the standard of discipleship is the same in this world and in the next.
In principle it should be neither easier nor harder to exercise faith or to repent in the spirit world. Were that not the case, those in that estate could not be judged according to men in the flesh. For some it will be natural and easy to accept and live gospel truths, for that will have been the practice of a lifetime. For others it will be very difficult to do so, for eschewing the things of the Spirit will have been the practice of a lifetime. The difference is not in the gospel but in the hearts and souls of those to whom the message is being presented. We must allow, however, for circumstances in which people were prisoners to experiences in this life that prevented them from having a fair chance to embrace gospel principles here. When they are freed from those bitter chains, many of them will seek the blessings of the gospel." (Straightforward Answers to Tough Gospel Questions, Joseph Fielding McConkie)
What you might be refering to is the following scripture from Alma:
"Ye cannot say, when ye are brought to that awful crisis, that I will repent, that I will return to my God. Nay, ye cannot say this; for that same spirit which doth possess your bodies at the time that ye go out of this life, that same spirit will have power to possess your body in that eternal world." (Alma 34:34)
This is directed at those who reject the gospel in this life thinking they can wait until they die and see what's on the other side. However, just because someone has now died and entered the after-life, conditions will be such that, even though they now know that there is in fact an after-life, it is not necessarily going to convince them of the necessity or truthfulness of the Gospel of Christ. When they die they will take that same spirit and personality with them that they possessed in mortal life, which will be a great influence on them in the next life. There are also many who have experienced a "spiritual death" while on earth due to a willful disobedience to God. This will also effect their ability to accept the truth.