I've made several posts about it. If you're desperate, go on my profile and look at my posts. I go more in-depth with the concept in older posts.
Save Game Manager GX extracts save files from your Wii. So the first step is extracting the save onto an SD card and loading that with your PC. The folder it's in will be a bunch of numbers with a letter at the end. For example 9004850486245e.
Inside that folder is the files that the save uses. Your goal is to remove any friend code data from the "completed" save and put in new friend code data. So you need to start a new save for TvC (delete the old one) and extract THAT one to your PC, too. Since it's in the same folder you need to keep them separate somehow - add onto the name of one folder to be "completed" and the other "new FC" or something. Just keep the original file name because you need that to install the save again.
Look inside each folder. If you're lucky, there's seperate data for your unlock progress and for your Wifi data. If so, just swap in your new Wifi data with the old unlock data and install it again with SGM GX. Otherwise, this might not be an easy process and could take a hex editor like MadEdit and a lot of trial-and-error. It may not work at all. The game could employ checksums, which you probably don't have the skills to reverse-engineer (neither do I).
Good luck.