Have you got some screen grabs of the before and after? Who did the redesign? @jeans
You are right that you can never really tell how a design will go down, and there are many factors that play a part - but often you can take these into consideration. Even something like browser compatibility can have a significant impact, which can then have a knock-on effect, particularly if your site no longer works well for a large enough number of regular members.
I think when moving to a new platform, the design either has to be sufficiently (/radically) different or as close to your existing design as possible, even if this means the overall branding or look and feel. Ideally you need to judge on a per site basis tho.
Discourse needs a couple of killer features to make it fit into the radically different category imo (as per some of the suggestion I have posted).