@indycutie The short version is, move everything you want to keep here:
C:\Users\Public
And retrieve it once you're in the new account. This folder can be accessed by opening a File Explorer window, clicking This PC in the left panel, double-clicking the C drive, and opening Users.
Please do test the game in the new account before transferring any content. If it doesn't work even then, stop there and let me know. Otherwise, add your old content a little at a time so you find out right away if there's a problem.
If you add your installed content, not the downloads but the files in DCCache, InstalledWorlds, etc. to the new folder, and the content doesn't show up, it's best to remove those files and install everything fresh.
Here's a description of everything in your user folder:
https://sims3.crinrict.com/en/2011/01/faq-user-files.html
You don't need to transfer any files or folders that the game will recreate on its own, and in fact it's better not to when you have a choice.