I don't know if anyone can help with with a decision I have to make. My very first original site, WhisperWillow.Com is currently at version 9.03.00 and I really need to upgrade it however; I have such massive changes and mods that I did along the way that I'm afraid I will break it. I really want to keep the theme mods that I have done which are extensive. Please take a look at my site and give me some ideas/pointers on how to accomplish this. Unfortunately I did not document all the changes and doubt seriously if I can remember them all.
https://whisperwillow.com/

If you've only edited theme nothing should break. From that link you provided I have no clue what you modified in core. I saw only many modifications in Atom-X theme.

Anyway I always say that modifying core files is not a very smart idea. Since v9.10.00 we have auto updates system so modifying will be even worse. Because updater just replace all files and runs update scripts (upgrade/ folder). Btw in v9 you can do many modifications on theme level.

Thanks for your response RobiNN. I will have to investigate further to see how hard it will be.

To find out what is changed, compare your site with the original installer version V9.03.0

Thanks douwe I should've thought of that myself but didn't. I will proceed.

Well thankfully I backed up my db and site files. I tried the full upgrade to 9.10.10 and it totally blew up in my face. I had to restore both. Forget this request I'll stick with 9.03.00