The question is a good one because it fully describes the actual dilemma that takes place. You have the integration of two souls, and their connections to the akashic record, attempting to fit within one physical being in the expression of the body and all that it does. This creates a complex dynamic and an interplay that will require a fair amount of learning to acclimate, and to work on adjusting things to set things right, to seek solutions for the unfinished business of the prior occupant to the extent possible, if only to not be unduly burdened. Many times people surrender the body when they are at a low ebb because of all the past trauma becoming too much for them. This will still be there and waiting for the walk-in, who must find a way to deal with it, in order to prevail and find a way to make things better ideally. In most cases, this will be manageable because the incoming soul and its prior experience will be quite different, and so there will not be the ownership of the shock and trauma of that other person, and it will seem more remote, and not so very troubling, because the walk-in will not resonate as strongly with those issues unless there are shared vulnerabilities and weaknesses. In that case, there may be an adding to the issues of the walk-in they bring along. The walk-in will connect to the akashic records on its own energetically, in the same way the departing individual connected in the first place, through the soul component present within the body to complete the incarnation process. The walk-in will connect to their akashic records, but will be influenced energetically by the subconscious of the host body still connecting to the akashic record of the prior occupant. So they will have, in effect, a double dose of karma to deal with, and of course because the departing soul was often troubled, and wanting to leave for that reason, there will be initial difficulty and burden faced by the walk-in, and this may become quite a compelling challenge, and will be the first order of business in any event, and may well require major healing to surmount this in order to accomplish useful work which, after all, is always the purpose of an incarnation, to take on a challenge and accomplish things for betterment.
So this is, in many ways, an unnatural state of being but can be a blessing for all concerned. Giving up one’s body to another can be an acceptable escape for someone who is too overwhelmed by the negativity they face. So they can return early to the light, and have a period of rest and recuperation before resuming their next incarnation. If they are too vulnerable and have a severe health consequence, this will further impair things and add to the suffering and the next lifetime will bring even greater challenges as a consequence. Sometimes it is safer and better to depart than to continue on a downward spiral and add much additional trauma to the backlog of unmet need. When this is managed deliberately, to be a benefit for the self and to serve another, in allowing a newcomer to have an opportunity to incarnate and have a jumpstart, in a sense, from avoiding all of the dependent years, this can be a blessing, even though there may be a mess to contend with from the outset.
Each person has their own personal mess, from many prior lives of difficulty and trauma. So this is not necessarily an intolerable burden, by any means, but something that can be dealt with. After all, only those things of consequence to the current life are of an urgent nature, and a pressing need in the priorities. So this can usually be managed by a fresh perspective brought by a walk-in. And that is why the process is allowed. It can be a win-win for both the prior owner of the body and the walk-in, as both will benefit from the exchange.