Hi! If I change my username does the “join date” change or is there an indication of what my old username was? Thanks!
The join date doesn’t change. While AFAIK there’s no record available to members of what your former username was, it tends to be easy for anyone who might care to figure it out.
To get a change, e-mail (or PM?) one of the board administrators (TubaDiva usually handles it, but AFAIK any of them can do it). The working rule here is that members are entitled to one change of name, more or less at will, and any additional change is at the directeion of the Administrators (and usually requires an excellent reason for the second change).
This is from memory; IANASDMBAdminOrMod. There may be something in the FAQ on this, which is probably more reliable than my memory.
Moved from GQ.
As Polycarp indicates, the join date remains the same. While there is no direct indication of the previous username, it can generally be found out by looking for posts in which you have been quoted (the original username will remain in the quote), or where posters have referred to you by name in posts.
The way I’ve seen it work most effectively is if the poster stops posting for a bit so that every thread they’ve posted in falls off the appropriate pages. And then they go for a new name.
Thanks, all! I appreciate it! 
Thank you, Jenny! 
discretion. I usually don’t do this, but in this case that was so horrible a typo, and appeared to be one for the wrong (but possible) word, that it needed fixin’.
Took me less than a minute in this case. Not that I really cared, I just got curious how hard it would be