ROM Hack I need help I'm a newbie with programming

[xgh0stx]

I recently got a Nintendo 2ds xl and installed Pokemon ultra sun. I tried to install a new sd card because I couldn't update my game ( lack of memory). So I decided to buy a new sd card 32 gb. I found out the fat32 file system won't accept my Pokemon sun app info because it was larger than 3.5 gbs. So I copied and pasted the info. Reinstalled Pokemon Ultra sun, and found out after I reinstalled the game and replace the sav file on my sd card on the new sd card it says this save is not compatible with this version. So I decided to go back to my older card and now that says the same thing. I have been researching and found that if you have a cfw like fbi you can fix this error by surpassing the check sum error. I don't know where to get the program for the FBI or how to actually manipulate the file. I would be willing to offer compensation for someone to teach me this, ( I'm not sure if this is allowed). I'm not losing the 70 hours I've put into this game, my time is way too valuable and I can't put that effort into a game all over again. I managed to catch my almost ultimate dragon team. All I was missing was dratini. Any help with how to achieve my goal would be helpful. I also have a MacBook, but I can use a chrome book with windows or a desktop pc or if I get ambitious fix my charger port for my pc laptop. I've never messed with actual programming like this, I've only dabbled in c++ a little.