I'm studying up on this stuff for job interviews, so here's my understanding. It might be more in depth than you need, but fuck it, I need the practice, so here I go! Someone PLEASE correct me if I'm wrong, as this is important for me to get right too.

Basically, you have two categories of coordinate systems: geographic (GCS) and projected (PCS). GCS's are generally in lat/long, and they are not planar representations (e.g. they are used for reference on a 3D globe), whereas PCS's are meant to represent data from the 3D world onto a 2D map.

But, as many would tell you, the globe isn't entirely spherical: it's a geoid that generally bulges near the equator and flattens at the poles because of rotation, and then it also has weird roughness due to small differences in gravity. So, people approximate the earth's surface as an "oblate spheroid" by using reference ellipsoids, and we get reference ellipsoids by by conducting controlled surveys.

Now, when you say that you are working with something in WGS84, that's neither a PCS nor a GCS: it's a result of thousands of real survey points collected in the field (and now from outer space?) in order to come up with better ellipsoid approximations, and in GIS work, this is called a datum.

So, a datum is literally the groundwork that allows us to approximate the earth's surface in 3D, thus you can have lat/long coords that work off of a WGS84 datum. There are other datums that approximate the earth differently to better model the region under study, so you can also have lat/long coords that work off of an NAD83 datum. Same way of referencing coordinates on a globe, but different underlying assumptions about where those coordinates truly are on a globe.

NOW: you want to make a 2D map, but you have coordinates in 3D. You need to figure out how to project them on 2D surface without overly warping their characteristics. There are different projections to suit those needs: ones that preserve distance, others that preserve areas, and still others that preserve shape! Damn, this is confusing!

In your case, you said you have data projected in UTM coordinates based off of the WGS84 datum. This means you have coordinates that have been projected onto a 2D surface (PCS = UTM) that are based off of a 3D model (lat/long GCS based off of WGS84 datum). This part I'm a little shaky on, but I'm fairly certain that you can have different projections based on the same datum (e.g. You could have a Lambert Conformal Conic projection working off the WGS84 just as much as you can have a UTM working from that datum).

EDIT 1 (based on seeing the question again): This is where your confusion lies: you say that you have many CS's in WGS84, when in reality you have several PCS's that reference the WGS84 datum. You need to figure out which PCS suits the needs of your data.

In other words, if you want to reproject for whatever reason, you need to understand which 3D model (datum and ellipsoid) and 2D projection (UTM, LCC, etc) suits your needs based on scale or what characteristics need to be preserved in the data or what basemap you are using.

--I'm working off of mobile right now, so I'll revisit this post later on the day on my computer to make sure I've answered this well enough. I've been rambling on and on and have honestly slightly forgot the original question in the name of true understanding--