The IE8 setup procedure should check your existing plugins and carry them over. There is always the possibility that 'some' plugins are not compatible, but IE8 has been out for so long now that they will have been updated.

No need to touch Nortons. It should recognise the new browser. Flash and Java may need reinstalling but they are easy to do if necessary, and you will soon know when you visit a web site that requires either. The web page will tell you.