Sorry, I can't answer the question about the number of posts, except to say, it sounds like there is some sort of glitch going on, and hopefully the correct number will be restored when the techs figure it out.
We allow a single username change per member. Please do the following:
1) Make sure the name you want is available by doing a search for the username you want. Type your choice in the general search box and click search, then look for any "User" results and click on that. This will show any names that are the same or similar. If no "Users" come up, the name is not taken.
2) Make sure the name is one you will stick with permanently.
3) Contact any moderator or admin with the username you want
4) Once your name is changed it is a good idea to post a note in your signature or elsewhere to let people know... this is especially needed for members that have been here a long time.
Note: Usernames must be 4-15 characters, including spaces, and should contain only alphanumeric characters and single spaces between words.
Cheers!