I hate the vanilla fat morph with a passion. I realize that there's plenty of amazing CC fat body presets out there, but I want to know if it's possible to cut out the middle man and replace the bodies (morphs? meshes? presets? no idea what specifically needs to be changed) so premades and townies are automatically affected. It would be the same UV and everything, just a slightly different mesh. I've only done texture mods so mesh/preset modding is way out of my comfort zone, but if anyone is able to explain how this works or wouldn't work, I would appreciate it.

Still wondering if anyone knows how to help

The limitation here is that there is no universal body mesh to begin with- really, a sim is a floating head that wears clothes that have the body pieces included in them.
For that reason, there is no replacing the body or the morphs being applied to it unless you'd like to embark on the project of applying your changes to every piece of clothing in the game. Not impossible, but perhaps biting off more than you wanted to chew?

A default replacement preset is less certain. I don't know if premade preset data is persistent, if it would reflect any changes to the preset or stick to the old appearance. If we assume that it would work, it would then still only affect premades who actually used the preset. There are likely fat sims who used the other presets and simply had the weight slider increased.

It's tricky, with no perfect solution, but you could definitely tinker around. You could start with making a new fat morph for the default nude underwear set- you might find that you like the process and actually want to commit to converting more. Or make a sim with one of the Maxis presets and test if replacing it affects them or if they need to be taken back into CAS for it to update.

You might be interested in this mod for the townie part: https://simplifiedmodding.tumblr.co...m-randomization
You'd want to enable your preferred CC presets for random if they aren't already, in combination with the above mod.