So I'm not terribly familiar with the default replacement process but I wanted to get a quick answer to this question before I started going about things the hard way.

Say a simmer makes a default replacement mesh for, say, the afshortslick hair, and I go "oh I like that hair, but I think it would look better replacing the fhairsweep mesh." No other changes but which mesh the replacement is associated with. Is this possible, and, assuming it's not as tedious as starting from scratch, how would I go about it in SimPE?

You'd most likely have to change several resources, because depending on which method used, it's most likely the PropertySet that decides which hair is replaced. The mesh is linked to the 3DIR and doesn't really replace the mesh.

Tutorial on PropertySet hairs and clothes:
https://le-plat-du-jour.livejournal.com/56464.html

With this method, recolors of the original EA mesh should in theory show up without issues (most of the time, anyway), because the original mesh or recolor files aren't actually replaced. It's just the recolor PropertySet files that's replaced.

The "plain" method where the texture and mesh is replaced can have a lot of bugs. It's still used for some things, but doesn't work well for hairs because of the mismatch of groups.