Upgrading advice needed

I am looking at upgrading to the new version of WordPress but have a couple of questions.

Firstly; I have the Enforce www. Preference plugin installed on my site. If I upgrade, will this still work as normal or is it something I would need to get rid of?

Also, if I copy my blog folder completely onto my PC before upgrading, if something goes wrong, can i just upload that folder back so everything is back to how it is now (just in case)?

Thanks in advance