Upgrade old html to html5

Question

Does upgrading old html to this version html5 means only changing doc type declaration and refactoring code if there are elements that are not supproted in html5?

Answer 1

What about XHTML, is upgrading the same as described above?

Answer 2

At moment we are using xhtml, but client wants face lift of the app, so we are considering to include middle version of bootstrap due to limitation of jquery version (1.2.1) and moving to html5 to simplify support. What are your recommendations, should we proceed with our plan? Web forms app, .net 3.5 and 2.0..