1. Planning - it sounds a bit obvious, but you need to make a plan before you decide to implement a large change. If you don't consider all eventualities then you may well find you have to fix things at a later stage, possibly at great expense. There is bound to be something that you do miss, but by planning well, hopefully it won't be anything too big.
2. Don't expect everyone to be enthusiastic - on the drive over to my mum and dad's my son declared "it's boring to see the same people all the time!" Of course what he really meant was that he wanted to stay at home where everything is familiar as it's hard to know what to expect when you are only 4. People are inherently resistant to change and whilst it may improve your organisation, not everyone will view things with the same perspective as you and they will become nervous about the unknown. I recommend letting people know as soon as you can what it will mean for them, allow them to ask questions and accept their input where possible.
3. Try to minimise the disruption for everyone - not only are the boys unsettled over the change, I also have to consider my poor parents who not only have to share their house, but have also discovered the noise and mess levels in their house have increased by about 300%. Again, communication is key when minimising disruption. Do your supporters really care that you are moving office or have a new computer system? Not really, however if you are clear about when any downtime may occur and how long for and wherever possible have a back-up system in place, hopefully you won't disrupt them in their experience with your charity too much. Honesty with supporters could even lead to stronger relationships with them and therefore more support.
4. People might still get scared - you can help them through it. My 1 year old was coping pretty well until I tried to bath him at which point he freaked out! Even those most enthusiastic about the change might have a wobble halfway through, and when scared, people often become defensive which can be difficult to deal with as a manager. Take the time to listen to your staff and be empathetic about their concerns. You might want to consider some time away from the office as a group in order for people to relax and open up.
5. Have a 'change champion' - this is someone who knows all the details about the change, who it affects and how, timings, potential pitfalls, how the organisation should look/work afterwards and how to tackle any problems. They are there to answer questions and ensure things go as smoothly as possible. Depending on the size of your organisation you may want multiple people to take on this role, and make sure everyone knows who your change champion is.
When going through big changes it is inevitable that you may lose some people along the way (metaphorically or literally), but if you try and consider all these points you should have a smoother ride and keep those around you happier. Once it is all done you can lie back and relax in your new bath/office/way of working, and enjoy the fruits of your labour, which is what I intend to do when I finally get my bathroom and house back!