Hard or Soft, How Do You Launch?

There seems to be a new approach to launching a website redesign. If you’ve been following the “Yahoo”:http://www.yahoo.com/ home page design since February you’ve probably seen their new site and had the option to swap between the new and old designs for several weeks.
Yahoo has only recently started forcing moving users to use the new interface but they still allows users in Canada to chose. They’ve been maintaining the two designs for a while and their home page isn’t the only example, they’re doing the same in the Yahoo Mail interface. Users can choose the flashy new AJAX interface and the old mail user interface.
Yahoo is not the only company following this soft-launch approach for site designs, Microsoft is doing the same with their MSN/Hotmail Mail and the new Live Mail Beta interface. Both mail products bring forward all of the user’s email, contacts and other personal data but just introduce a new user interface.
On the other end of the spectrum is the new “Digg”:http://www.digg.com/ v3 site redesign which took the hard launch approach. For weeks many of the top stories on the site related to the new design and how users want to maintain the previous look and feel. In some cases “Grease Monkey scripts”:http://darcasey.googlepages.com/greasemonkeydogg were developed to help re-create the older site design look and feel.

All of this points to a rather interesting question about site redesigns and how they should be launched. The hard launch approach, wherein a new design is launched and old design is forgotten, may not be the best approach for design launches. I haven’t seen any research that indicates a redesign is disruptive for the user but my own experiences have been limited to much less complex sites. _(editors note: “Jared Spool just republished an article called The Quiet Death of the Major Re-Launch that addresses this same topic)_
Some key questions need to be answered:
* Is the soft-launch approach better to accommodate users’ transition?
* Are users becoming so attached to existing interfaces that they can’t handle a hard launch?
* How does this effect the sites maintenance going forward?
* How long should the old interface be maintained?
Overall I think all of the newer versions of the sites mentioned above are vast improvements but as dependency on the user interface increases it may be harder and harder abandon old designs.