I apologize for the late reply! I have been away from my computer for the past week because we've been working on our recruitment stuff! (which I'm VERY EXCITED about!!!)
First off, don't freak out because you're a junior! I actually joined my chapter of ADPi last Spring and when I joined I was a junior. Like you, I was also a transfer student and worried I was too "old" for the system, but it turns out I wasn't. I'm so glad I didn't let my fears of being "too old" stop me from rushing, because I have definately found the place for me!
I don't really know what the other chapters' policies are on upperclassmen, but I have met women from other chapters on our campus, and they were all so great!
Congratulations on your upcoming transfer to UNT!! I hope you find your home within UNT as well as our wonderful Greek system!