Just thought I would add my experience to this. We are a new single academy, and had been running Arbour from when the school started. That seemed to work fine for the small school (and without any IT Staff). I started as ICT Manager last January, and oversaw the migration to Bromcom over the summer.
For the migration from Arbour, we had to write a load of reports to get the data out in csv format, and then work it around into the format Bromcom needs for the import, which did take a while. That was made more complicated by the slightly haphazard way we had been using Arbour, but we got there. We then uploaded all those exports to Bromcom and over the course of a week or so they worked through importing them into our system, and coming back to me with questions! The only real quirk I remember from the migration was around teaching groups - the import brought them all in with the same name and description (eg 1MaA), but as soon as you when in to edit them, the "Proper" description (eg First Form Maths Group A) started to appear. Other than that, it was all extremely smooth.
We are now nearly a year into it, and it has been for the most part very successful, we are still struggling with the behaviour module a little (more to do with us really thinking through how sanctions work - if you give out more sanction than there are slots in a week, they will start to book up way in advance for some of our kids!).
If you have more questions, please do ask!
Tom