It seriously sucks, there was allot of information on that account that I actually use lol. Like how to get the flex src for cam4btc to compile so I can relaunch the cam site. That's actually how I figured my account was hacked.. I was looking back on some older messages because I am having problems building the source so I can change the ip to a new server.
Now i'm kinda screwed. I guess that's a lesson learned always keep hard copies of important messages and docs...
Honestly exactly what went on with your account is probably not going to be determined (but it could well have been hacked and that does seem to be the most likely thing).
The person now in control of the account has basically admitted that they did not do any due diligence about said account and have realised that they have screwed up (although they are yet to admit it).
Unfortunately if you don't have a way to prove prior ownership of the account then there is really no solution here so perhaps you'll need to accept your new forum name (or create another one).
It is understandable to be upset over losing a name but also you need to realise that your password must not have been very good (otherwise your account would not have been hacked).