Henryson will beta 8 affect the previous version if already installed
I assume that the core of existing forums will have no problems to update at all. But before updating you will have to check whether the extensions you use are already available in a compliant version with the new core. And if you have written your own extensions, you will have to make them compliant yourself.
I have set up a Flarum forum myself. I use some core extensions and some popular non-core extensions which I both expect to become compliant quite soon after beta 8 comes out. Furthermore I have written some small custom extensions myself which I will have to adjust. I will first update my local test forum which I will feed with a dump of my online forum, and once everything will be in place I will switch my real forum too. This should be a pretty safe aproach.
