We haven't done it (yet), but some people are asking us what would be the impacts of installing wikifab's extensions on top of an existing Mediawiki site.
If you plan to do it, here is what you should now:
the styles of header and footer will look like ours (but the links can be changed)
user profil pages will also look like ours
all your existing pages will remains, but the style will be a little bit different: for example this page will look like this one (you will lose the left sidebar and the title font will be a little bit different).
Appart from that, your wiki will remain the same and you will be able to enjoy Wikifab templates and tutorials.
You will also be able to put anything you want on your homepage (it won't necessary have to looks like ours).