Well, generally most of your layout will transfer on migration. There are a few things that may not be able to translate well, but it should pretty much...
Generally most people are advised if they have a test board at Yuku to get the skins ready to go before migration. That way if, for instance, the sidebar has a little bit of a conflict you already have it sorted out, otherwise, you can always go to Skins and CSS for any problems or questions that come up during migration.