I've had some that have stayed just about exactly the same, and some that changed drastically after they went through chameleon puberty (both from mellow to aggressive and aggressive to mellow and various levels/mixes in between.) There have also been a few that changed during puberty and then went back to normal after a few months.
I had one (my favorite panther who has since passed away) that was very scared and timid when I had him. He was that way until he was about 8-9 months old and I only ever handled him when I had to (so maybe twice a month max.) One day he just decided to hop onto my arm and go for a spin. After that he became very friendly and liked to try and grab my arm whenever he could or even crawl down off his free range and find me if I was in the same room.
I also have a male veiled who has always been very aggressive and disliked me and would only tolerate me when I had a feeding cup in my hands. A few weeks ago he had to have his tongue amputated and now he is much more mellow about me since he can only hand feed now and is offered water through a syringe daily. He still doesnt like being handled, but he doesnt gape, hiss, lunge, or bite at me unless I am poking and prodding him. He also got a little less vigorous with his attacks/warnings once he turned 4, but it wasnt by much.
Other than those two, the others haven't had as drastic changes.
I don't have long experience owning panther but from what I know one of my male panther just turned little bit more aggresive towards me as he grows older.
I am guessing the adulthood issued as he's more teritorial and perhaps 'horny'. forgive the language.