Both the OP and your reply emphasize the what/why... are there any resources available explaining how? If not, it sounds like this is a great opportunity for some blog posts!

Along the line of "you won't have time to edit eveything" is you should have some form of legacy rendering, so that your old images/assets can look "decent" in the new layouts/containers without actually updating everything. This might be as simple as providing alternative layouts for old image sizes, etc...