Planning is the most essential thing to do. Include this steps in your plan:
You need to anticipate the outcomes of each of your actions. In order to assess these, put yourself in audience perspective. If you are one of those followers, what would you think or feel if the administrator of that brand do this, or that?
Ask the people for their ideas. Posting here is one good example.
If you plan to migrate to another domain, notify the readers. You can make this by putting announcement box in the header, sidebar or any part of you site that is visible enough for them to see. Notify your subscribers via email (if allowed) and through social media.
During the actual migration, don't shut your old site or the domain, instead redirect the visitors into your new site (301 redirects maybe). With your new website, for the first few weeks or even months, inform them that this brand is from this brand or website.
For social media, twitter is easy. You can just change the username for your vanity url and also the account name. With facebook, fan pages is difficult to change, or not even allowed, after 200 likes. The best way to do is to have another page. Inform the followers (likers) of your new page via post. You can also edit the info box of your new fan page URL. If possible, change the cover photo with announcing the new fan page. Don't delete your old page, this mus be an asset for not all have followed the other one or this might serve as bridge.
And lastly, research more if time allows you. Always think one step ahead.