Sorry about your brain injury. I forget a lot of stuff and don't have one (well that I know of anyhow). There's a couple of easy obvious answers to your question. #1 You need to change the top box, (possibly) reel glass, and belly glass for the new theme. You'll also have to change the reel strips (pay attention to strip order if they are not all the same). That will make the machine look like the new theme.
Next you'll have to change the Stepper base chip for the new theme. You most likely will not have to change anything else. When the game boot up you'll get a couple of error messages, but a couple of turns of the jackpot reset key and maybe a 2-3 second push of the MPU self test will get you through though that.
Some more steps may be required. How about posting some pics of the machine and what you're trying to change it to? That will help us help you.