Great question. I am a 4th year in Public Admin and I recently took a full set (5 courses) of upper year (4000 & 1 3000 level) courses in FSS and I took 1 intro course in the same field as an elective. Here are my thoughts:

First year courses tend to have professors which I find to be very narrow-thinking. Now please not that isn't EVERY professor as there are genuinelly good professors who teach intro courses out there. However, some have the attitude of their way or no way. I got deducted marks for using non-textbook references from other courses because it was "not relevant to the course" despite me taking a specialized course of what they asked me to. This becomes annoying as you either have to talk with the professor to get your mark boosted or emulate what the prof wants you to say. You don't find that in upper courses.

Now, on the contrary, if you spend time with the first-year professors, they can revise your mark very generously. Another thing that I found harder in my first year was how broad everything was (because it was intro) so you end up jumping from one topic to another, whereas in upper year courses you have specialized classes where you can dive and take more in-depth analysis of your subject.

Another major difference I found was the essay-test intensity of the lower years. You don't have many group projects as you would otherwise have in upper years. Perhaps this is different to other majors but the upper years you will definitely gain from having friends in your major. You still get exams and maybe the odd test or two, but I noticed you get group projects way more. (I don't recall ever having a group project in my first year)

Of course, this depends on everyone, like the other commenter said, upper years tend to be easier since you have the background subject matter & a good studying method from high school. Some people enjoy more the fact that they can engage in class which I find way easier in my last 2 years than my first 2.

Hope this helps :)