Are you migrating an episerver site to a new version or are you going to re-build a site based on an other platform to EPiServer?

If the latter, you should, IMHO, take a general grip on the site as if you were creating a completely new site. Go back to the initial requirements and take them for another round, dropping old requirements and adding new ones. If you try to do a 1:1 migration you are bound to run into problems with old designs that were affected by the old architecture and have to be really squeezed and twisted into the episerver platform. It will only end in tears. Promote this as a chance for the customer to improve functionality and user experience. There are, IMHO, no pure technology migrations.