All right, so I was fortunate enough to get a gaming computer for TS3. Everything moved over to the new computer just fine, except that I didn't have a hard drive of TS3, and a game code instead. Which meant that, upon loading it into the new computer, it patched at 1.69. So obviously there was no other option but to download the Superpatcher for 1.67. I was feeling particularly pleased to have found a fix, until..
"The Setup has checked that no version of The Sims 3 is installed. This update requires a previous version of the application to be installed."
Since there is very much a version of TS3 already installed, I turned to search my problem on google and came up empty handed. So here we are.

System Specs:
(There are two different graphics cards, I wasn't sure if I should include the specs for both of them or not. I did, but call me out if I did something wrong.)

If it already patched to 1.69, you are not going to be able to use the superpatcher to patch to 1.67, because it's going to find the 1.69 patch, recognize that this is a higher patch level, and refuse to do anything to it.

The only way to downgrade your patch level is to install from a source that is not already patched to 1.69 (i.e. older discs) and patch that installation. Sorry.

Thanks, @Ghost sdoj. I thought I had read somewhere that I could use superpatcher to downgrade, but I guess I read something wrong.
Will 1.67 mods work with 1.69? All the ones on MTS?

Some will, some may not. It depends on the mod.

If it's a scripted object or an XML mod, it probably will. No guarantees, but it's likely that nothing it depends on has been changed unless it's something affecting the Launcher.

If it's a core mod it won't. A core mod is built to a specific patch level.

Well, it is good to be sensitive to that awful sounding error message core mods can kick up, I would hate to see it at launch each time. But...

If a core mod was designed to run on 1.67, it will display the core version mismatch error on startup if you have 1.69. But if that's the only reason for the error message, the mod will still work the same way once you click through that. There is nothing in any of the three releases of 1.69 that changes gameplay, only the build number (hence the error message) and the way that the game launches.

Script and tuning mods are not affected by the new patch at all.

The only TS3 core mods in common usage, that we know of (or that I am willing to talk about), are:

NRaas ErrorTrap - updated version available, we will continue to develop future versions for both 1.67 and 1.69 in parallel releases
NRaas UntranslatedKey - no updated version yet, but this is mostly for developers
CMarNYC's XCAS - updated version available, the 1.67 version is as well on her archive page
AwesomeMod - no update, they appear to be ignoring the whole thing over there
Consort's Grow and Fitness mods - no update, but he never updated for 1.67 so this is nothing new

Out of the so many thousands of mods and scritped objects out there, that should really be about it.

Thank you both for the info! Highly appreciated.