Hey everyone! As the title suggests, I'm new and needing some input from those who have more experience with bikes. I've always loved biking, but due to the financial strains of being a college student, I haven't updated my bike in too long. Now it's all rusted from when someone stole it for a few months and my knees hit the handlebars...so an update is due. The problem is that since I haven't gotten a new bike since I was 12, I have no clue how to choose one.
I ride almost exclusively on bike paths. They're mostly paved, but there are some long treks of gravel, so I've been wanting to get a hybrid. Since I'm still in college, my budget is limited to $500, so I've been trying to decide between a Trek 7.2 FX and the Jamis coda sport. From what I've read on some of the forums, they appear to be rather equitable, so I'll probably end up choosing whichever one feels better on a test ride (I'm open to discussion on the quality of their components, though). However, what is really throwing me off is my cousin has a Mongoose Switchback A2 that he claims to have customized into a road bike. First of all, I can't find anything on the internet which indicates this bike even exists other than a random picture on facebook. Secondly, is this kind of customization even possible? Also, from what I understand, Mongoose isn't exactly known for its quality. However, my cousin claims that the only stock part in the whole bike is the frame. Is that enough to make a crappy bike decent? I'm trying to decide if I even want to bother seeing it because he lives three hours away from me.

Either of the bikes you mention would probably be fine. For similarly priced bike shop level bikes, there will be some variation in components selected, but very rarely anything that is critical.

I can speak more to the Trek, since I ride a 7.3 FX, which I use with 35 mm tires (this width I think comes on the 7.2 stock). It handles pavement and gravel fine for me. I suspect the same could be said for the Jamis, but I have no personal experience. Your approach of picking the one you like best on a test ride is valid.

If your cousin bought the Mongoose and then bought parts to upgrade or change it, then the chances are he paid more than he would have to buy an equivalent bike new... I wouldn't expect to be able to upgrade a cheaper bike to the same level and save money unless I already had many of the parts.

While a mountain bike, or other bikes, can be made more road worthy by swapping tires and making a couple of other modest changes, it is still a mountain bike with different tires and a couple of minor changes.

Yeah, I don't plan on trying to convert a mountain bike like he did. The reason I mentioned it was because he's offering to sell it to me, but I'm rather skeptical of it.

I can't say much about the Mongoose, don't have any experience with them, but I think why you're having trouble finding it is because it's a Mongoose Switchback AL (not A2). The L is kind of in cursive and looks a bit like a 2 on the frame. If it's in good shape, fits you well, and he'll sell it for super cheap then it might be worth looking at if you're really concerned about budget and want the cheapest thing that you can get. Otherwise I might go with something better.

The 7.2 FX and Coda Sport are pretty much equal bikes. In my opinion I think the 7.2 FX is the best hybrid bike for the money at that specific price point. That being said, ride both and see which you like the best.
Read this if you haven't yet thesweethome.com/reviews/best-hybrid-commuter-bike/
and this one about locks The Best Bike Lock | The Sweethome

The Trek and the Jamis are similar bikes and are both popular and well liked on the forums. You can't really turn a hybrid into a road bike and the expense of replacing all if the components one by one indeed is much more expensive than purchasing a road bike. Many people really like their hybrids and take them on long rides. Your plan of test riding both and purchasing the one that fits best is a solid plan. A new hybrid will be a magnet for thieves on a college campus so make sure you invest in a good lock. There are several threads about how to properly lock a bike to minimize easy theft.

Thanks for your thought, everyone! They're really helpful.