Windows 7 - Problems changing Home & Business key from Office 2010 Pro Plus

First off if this is the wrong way to post please forgive as I'm a complete novice.

I have a laptop that was legally covered using Office 2010 Professional Plus (via company MS action pack but is no longer legit.
I've probably caused the problem by trying to be honest.
I have purchased a DVD copy of Office 2010 Home and Business and am trying to NOT uninstall all of the Prof Plus for fear of messing up outlook and server settings. I have tried using add/remove program change key but it says I have an invalid key. I am guessing what it is saying it is invalid for Prof Plus. I have removed all of the applications that are not part of Office 2010 Home and Business.
Any suggestions please would be gratefully recieved.

You could export the files and settings from Pro Plus, then import to your new Office once it is installed:
Office Outlook 2010 Import And Export

Also, check out the 'Related articles' at the bottom of the linked page.