'What happens when you continue to have these conversations with yourself, and you find that over time what you want changes, but you are already knee deep into a poly relationship?'
My opinion (and it is only my opinion) is this.
In that case, you have to be honest with yourself and your partners about what it is that you want now.
And you have to accept that what they want may not have changed at all.
And then you have to accept that you have no right to expect or force them to change to suit your new wants.
And then you all have to talk and negotiate until you can come up with a solution which, while not being what everybody wants, is something that everybody thinks they can accept and live with.
Take, for example, the situation I suggested above, where a person has been very happy as one arm of a V while she was working, but wants monogamy once the deadline is met. Let's assume the person is a woman, in a hetero relationship. She has to accept that her partner and his partner are whole human beings with needs and emotions of their own involved, not simply there for her convenience. She *cannot* (or at least, should not) ask or expect her partner to give up his other relationship. It exists as a separate thing from her own relationship, and she has no business interfering in it, and no business trying to break up her metamour's relationship. She can, however, ask her partner if it is possible for him to start spending more time with her, and they can all negotiate that need together, but she will have to bear in mind that if he starts spending more time with her, he will probably be spending less time with his other partner, and that may be difficult for his other partner to accept. Or it may be that he will give up a hobby in order to spend more time with her instead. Either way, she should recognise and appreciate any sacrifices made, and especially any sacrifices made by her metamour. Honestly, as Antony said above, it would be better if she could recognise from the start what her long-term needs are, because then she could have made it clear to everyone from the start that she would need more companionship after her deadline was met, and negotiated that need from the beginning.
Or a triad situation. Say, for example, that there is a triad of one man and two women, and the man wants to end his original relationship. One dyad breaking up need not (and probably should not) result in the whole relationship breaking down, only the part which is not working. If the two women are still happy together, then there is no reason why their relationship should end or be undermined by the ending of the relationship of one dyad. If they have all been living together, then the situation is more complicated, but it should still be negotiated with the assumption that the women's relationship should continue. Perhaps they may end up living in two homes, with the point of the V spending half her time in each, or maybe they will have to simply try to get along in one home, making sure that each arm of the V has their own bedroom. Either way, each arm of the V will have to respect the other's space and autonomy if it is to work well. The woman whose relationship with the man has broken down will be in particular need of support if she would have preferred her relationship with the man to continue: he will need to accept that, and that his partner will need to spend more time with his metamour than with him, at least for a few months.
In all cases, I think it is important to remember that every person in the relationship is a whole human being, worthy of respect and consideration, just as we want respect and consideration for ourselves. It can be particularly difficult to remember this and to behave appropriately in times of change, when tempers may be high and there can be a tendency to demonise the person who is trying to push an agenda we don't want, but we should do our best. It is important to behave in an adult manner, and not descend to childish name-calling or abuse, sulking, drama, threats or one-upmanship. It is not a competition to grab what we can from the situation. It is not a win-or-lose situation. It is a co-operative venture to try to make sure that everybody's needs can be met as effectively as possible within the limits of the situation. Everybody will gain in some ways and lose in others, and nobody can expect to get all of what they want. To assume otherwise, and to resent not having it all, is to behave like a child who wants Mummy to buy her a cake and refuses to understand that Mummy simply does not have that much money right now. It comes down to justice, to doing our best to recognise other people's needs and to treat them fairly, and also being fair to ourselves.