Yeah, they got rid of the class distinction in the shirt department probably 15-20 years ago. They did away with the flat looking fish shirts and just issued everybody the upperclassmen "serge" shirts.
Personally, even though the fish shirts looked terrible compared to the "serge" shirts, I liked the class distinction.
As far as the OP's question, from the picture Canyon posted, that's obviously a non-reg. Look at the sideburns for starters. Why he thinks it's cool to wear a long-sleeve D&C fighter shirt, I have no idea.
Talking about non-regs wearing Corps uniforms reminded me of one time my Junior year, it rained and this guy in my Electrical Engineering class shows up in Zachary wearing black speedlace combat boots. I just smarted off, and said, hey, I like your boots. He then proceeds to tell me about how he was in the Corps for a week his fish year and when he punched out he kept the boots. They were useful for when it rains and for Bonfire.
And for the term "pop-up", they use that a lot here in Sydney for stores that are temporary. In some cases, they might last only one day. One of the most famous ones recently was a
one-day pop-up In'N'Out Hamburger store.
