Never change a running system is what you often hear, and it may, in particular, apply to already successful communities. I am sure many of you were confronted with similar questions like, why should you switch from vBulletin 3.8 if most of your members are happy with it? As an admin you may try to explain all the details, how vBulletin 3.8 was the last version of vBulletin that was up to your standards, that it didn't receive any feature upgrades in many years, that it's incompatible with modern software such as PHP 5.4, and so on. It sounds all reasonable; yet, at least in my experience, regular members usually have a different opinion about the need to migrate to another forum software.
When migrating to xF, one common "mistake" I have noted in various communities is that admins took an existing community with an established set of features (whether built-in or due to add-ons) and blindly upgraded it to a more or less plain xF installation. The result shouldn't come at a surprise: members complaining about suddenly missing features which they had learned to use and love over the years. No matter how much you focus on the benefits of xF, your members (in particular established ones) are likely to point out how the previous system was "better" because it had this or that feature that is now missing and that was, of course, most important to them.
So what to to do? For my own community, which still runs a heavily modified vB 3.8, I made a list of 60 or so features, some vB built-in and many custom, ranked from 1 to 5 (least to most important), that are presently not available in xF. Before I do the actual migration (which I hope will be around the time xF 1.2 is released), I am working through that list and complement the most important missing features with my own xF add-ons or with add-ons provided by the excellent xF community.
I am interested in hearing how you dealt with the migration process. How did you explain the need to migrate to xF to your community? Did you take time to prepare for the migration and explain it carefully to your members? How did you react to those members who don't like any kind of change and who focus on continuously expressing their personal disappointment rather than on the benefits that the new software brings?