Maybe, not quite yet… you are probably still waiting for the public beta to release… but when it does, this will probably be one of the first things you want to look at. While the Visual Upgrade feature in 2010 is going to be a nice way to temporarily see your upgraded site with your old branding, this mode is only meant to be used briefly while you learn how to actually convert the master page and other branding assets to 2010. Thankfully, there is an early release article on the MSDN that covers this process, check out:
Upgrading an Existing Master Page to the SharePoint Foundation Master Page
After reading through this list of steps, you may be wondering why you don’t have to change all of your Register tags at the top from 12 to 14 to point to the new assemblies… well lucky for all of us, Microsoft was kind enough to handle this for us with mappings in the web.config. While I think you should probably go ahead and switch to the 14 assemblies in your code eventually, everything will work perfectly for you if you just leave them pointing to the 12 assemblies for a while… pretty neat!