To create a new folder:
1. Rename your old sims 3 folder.
2. Launch the game and then go in and reset all your favorite options. Save and then, quit. (Two steps: save and then, quit. Not save and quit.)
3. Open the launcher and go to the downloads tab to see if the store update will trigger. If it does, install it. If it doesn't, go to an old save that has a contentpatch.package file in it to copy in OR go to your "my page" and click on games, updates, store updates.
4. Once you have the content patch, you are ready to reinstall OR copy/paste old files over.
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To reinstall: Go to "store install" in my siggie. It has everything you need to include spreadsheet and links. It avoids all duplicates.
To copy/paste: Let's start at the top of your old sims folder.
If you use collections, you'll want to copy and paste those over. Personally, I find them too much effort for the gain. You still end up searching through to find the folder you want.
ContentPatch should be done already. There is only one file you need. There is a do not bug me until next patch file too, but really, there will not be another store patch, right?
Current game isn't needed, but it is the data for the last save (which is also conveniently in your saves folder).
Custom music is obviously your music files that you may have added. So, yeah, copy them over.
DCBackup is a BACKUP of every file installed through the launcher to use for exporting. (These are only important to transfer cc.) THE ONLY IMPORTANT file in here, in my opinion is the ccmerged file, which makes your premium content work. However, these can go directly into your mods/packages files and you could then maybe weed out some EA files you don't like. Good luck knowing which file is what!
DCCache .ebc files are EA content installed through the launcher. They begin at 0 and number up as they fill up.
There are also .dbc files. These are for non-EA stuff you installed through your launcher, but sometimes, it's also venues (as your game might view that as an exchange download rather than store content). So, usually, this will be cc. I think missingdeps is for stuff you delete in the launcher that doesn't really delete. So deleting this file doesn't hurt. Basically, you'll copy over the whole DCCache folder unless you are wanting to weed out cc. THEN, MOVE out the .ebc files and load up launcher to see what's in those .dbc files.
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Still with me? If on a mac, NEVER delete caches! On a PC using windows, caches regenerate.
Downloads is where all your downloads files initially go. You should keep these files somewhere so you don't have download them again BUT... you should keep them somewhere besides the sims 3 folder in archive. Your game doesn't need them AFTER you have installed them.
Anything in your exports folder that you haven't uploaded, you may want. You might also want some of the files that you uploaded. AS with downloads, archive them outside your sims 3 folder. Your game will thank you for clearing out a bit more.
My IGA Cache stays empty. Experts have not explained this. No one seems to know its purpose. It is safe to delete it. I think someone created it and then never used it.
InstalledWorlds is what it says. You NEED these. Copy the folder intact unless you have worlds you downloaded and didn't like.
Library is for anything you ever put in your bin to include venues. Like the DCBackup files, good luck with that numbering system! DateTime is your best guess for what they actually might be.
Mods - you got this! There are the essential package folder and a file that allows your computer to read them (resource.cfg). You might also have an overrides folder that gives priority to anything in that folder over the other package folder. Copy whatever you want. Leave anything you don't really use behind.
Videos and Screenshots - keep them if you like, but again, not in the sims 3 folder. Move them out where you don't have to load them.
SavedOutfits is for anything you feel in love with while playing in CASt and saved. It you still want something, copy it over. Otherwise, let it go.
Saves: if you are like me, you use "save as" to avoid game corruption. You end up with more saves at the end of a session than you need. So, get in the habit of deleting the older saves after you quit your game, but keep one backup! Copy over the saves you want to keep. BUT, here's a great idea: Copy in the save you want to play - and only the save you want to play - before loading your game. Keep all other saves, like your downloads and exports out of your sims 3 folder. I recommend keeping them in the EA folder close to your sims 3 folder. Or on your hard drive or cloud drive too for backup.
sigsCache - this is data for EA about your system and login info. As they are cache files, they will regenerate if deleted. No need to copy over.
thumbnails - these package files can be deleted and will regenerate. I find it a hassle to wait for everything to be regenerated when I've deleted them, so I tend to keep them.
worldCaches - these again are caches. They will appear after you play any store world. The ones that come with EPs don't have them.
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Still there?
the *cache.packages you know can be deleted periodically as they can get huge. Anything over 300,000 tends to start crashing my game. So, if I've been CASt crazy, I watch the compositor caches and delete after I've quit for the day.
If you have userpresets, then, you'll want to copy that file over. This is all those things you saved in CASt like unlocked stencils as well as just favorite object recolors.