Change is hard. There's no two ways about it. Whether you are moving, changing jobs, re-configuring your family, trying to eat healthier, or any number of possible changes that can happen in one's life, change is highly stressful for most people.
Part of that stress arises from the unknown. There are likely going to be aspects to the change that were unexpected no matter how much planning has been done. For example, in the case of a move, even if you choose a safe neighbourhood, with a great school, and lots of amenities within walking distance, you might end up with a cranky neighbour. Since we don't know yet what we don't know, we often have vague worries and general anxiety about upcoming changes.
Another part of the stress is due to the inherent uncertainty of change - you just don't know ahead of time how the known change is going to pan out. Will you like it? Will you be able to do it? Will you be accepted? Will you thrive? Will the experience turn out in practice as good as it sounded in theory?
Change requires us to make adjustments to often well-developed habits. We have to create new routines to implement in place of the old ones in order to adopt the change and for the new situation to feel like the new normal. This is why smokers substitute gum when they are trying to quit - so they have a go-to replacement. If there are too many decisions to be made, this adds stress to our day but developing new routines and habits streamlines that decision-making and sets us up for success. Essentially, we need a new auto-pilot for many of the aspects of our new situation. If you are changing jobs, you'll need to develop new routines for how you get to work, how you schedule your workday, what you do at lunchtime, etc. Too many small decisions are taxing, which is why Steve Jobs famously wore a black turtleneck shirt everyday - to free up his brain capacity for more important issues. Once these initial routines are implemented, the new situation becomes much more manageable.
Lastly, change usually involves a loss of something. Even if the change is driven by you and eagerly anticipated, there will likely be some feeling of mourning regarding the loss or perhaps the loss of the potential/ the idea of what could have been. This can be a surprising and confusing feeling but it is normal.
Understanding the source of the stress is a necessary first step to be able to get a handle on it. Thereafter, some people thrive on detailed planning; some on meditation and faith; some on professional advice and guidance. I personally read every book and listen to every podcast that I can find on the subject that happens to be consuming my thoughts. It's crucial to find the balance that meets your needs in the face of our constantly changing world so that you can embrace change as it inevitably arises.