Hey Carter, welcome back to the forum~! There've been a few things added to the site this last year, like the +like function, so it does look a little different from your last visit. Don't really think I remember you though.. but that'll change now since we've officially exchanged greetings! :D
Great to hear you're back to stay this time, hopefully that means we'll be seeing you around a bunch! The forum's still really active so I'm sure you'll get into at least a few sections here, be it Pokemon or off-topic, depending on what you're most interested in. Speaking of Marriland, I actually used to watch his Let's Plays a few years ago (Diamond/Pearl I think) and it was pretty fun. Kinda miss watching, not sure why I stopped. D: Never really got into the forums over there though, admittedly.
And yeah, as Hikari said, sadly you cannot change your username. You are allowed to make another account though, just remember not to create a second introduction thread since you already have this one that's active!
Have fun and see you around! Make sure to give the rules a read since they've been updated a bit since then, and feel free to contact me if you need anything at all :3
