Switching to the next-gen skins
If you decide to switch to one of the next-gen skins you need to bear in mind that certain variables, in particular page variables will not immediately be available in your site. You are required to to copy and paste across the old variables to the new variables. It's a simple and fast procedure and won't take too long as the old variables are available to you - greyed out on the right hand-side of the edit pane.
Once you have copied the content across to an appropriate place, and after you have saved your page, you can delete the old variable if you won't be using it again. Watch out for accidentally deleting your page though.
Why does this happen?
The next-gen skins are structured entirely differently, and cater for new variables to allow for fine-tuned presentation of your content on a page and new features like multi-column pages.
For instance the page and sidebar variables are no longer present, but a left and right variable will be. It's up to you to run through your site and ensure everything is where you want it to be.
What about posts and photo sections?
Due to their relatively simple structure all content within a post and photo section should just work, although there are now entirely new ways of presenting this content.
What about logos and skin settings?
On the plus side your logo and other associated skin settings 'map' much better between the next-gen skins, in other words your logo should remain intact if you switch between these skins, and in many cases everything should just work.