Hi Rob,
Brave consideration you are making, although I am convinced there is probably more work in IT then there is in interior design. If you plan to go out on your own as a freelance designer the market is tough. With the financial crisis many people skip the interior design part and go straight to the builder. If you apply for a job realize interior design isn't paid all that well. There are still jobs available in sales. Kitchen, bathroom or interior design sales. Kitchen/bathroom chains which need you to assist clients in buying their kitchens/bathrooms. You do some designing with standardized programs.
I am not sure you will be financially better off if you walk away from IT. It's all a question of what matters more to you, your work or your wallet.