Other resourses helping you with the upgrade could be http://our.umbraco.org/documentation/Installation/Upgrading/general or http://our.umbraco.org/documentation/Installation/Upgrading/version-specific

The upgrade should not affect the look and feel of the existing website that is running under umbraco v 4.0.4.2. however, there will be a task of rewriting your old XSLT files to the new XML schema. The new XML schema was introduced in version 4.5 and it´s used since. Examples of the difference between the new and the old XML schema can be found here: 

http://our.umbraco.org/wiki/reference/xslt/45-xml-schema

http://our.umbraco.org/wiki/reference/xslt/45-xml-schema/xslt-examples-updated-to-new-schema

http://umbraco.com/help-and-support/video-tutorials/umbraco-fundamentals/xslt-in-umbraco-45.aspx

If you are most comfortable working with the old XML schema, it can done in the new version of Umbraco. Here is a guide to change the XML shema to the old schema in a new Umbraco version. http://our.umbraco.org/wiki/reference/xslt/45-xml-schema/switching-between-old-and-new-schema.

After you have updated to the latest version you get the opportunity to work with Razor too. I will link to some resources for learning Razor:

http://our.umbraco.org/documentation/reference/templating/macros/razor

http://umbraco.com/help-and-support/video-tutorials/umbraco-fundamentals/razor.aspx

http://umbraco.com/help-and-support/video-tutorials/umbraco-fundamentals/razor-recipes.aspx

And the team behind Umbraco is now redesigning the Umbraco TV, and there is a lot of videos about how to get data from Umbraco using Razor. But the new Umbraco is still in Beta http://beta.umbraco.tv/

I hope this can help you with upgrading your version of Umbraco.

/Dennis