As far as I know, registration date could be changed in the database, but it would probably be a major hassle. My old account said I registered Aug. 2001, but I have been around a lot longer than 2001; yet nobody saw me complaining about that. The thing is, that date.. it's just that, a date. If you really want people to know you've been here since July 2001 (or whatever you said), add it to your biography, your profile, or your signature.
Usernames can be changed; there's a few ways this can be accomplished.
Member numbers cannot easily be changed. As far as I remember, your member number is based on the ID number of your record in the users table in the database, which is assigned sequentially when you registered. Changing this number is possible, but since the database uses this field as the primary key value for the table, having large gaps in the sequence, or having the numbers out of sequence will really cause the database to chug. It's just really, really not a good idea.
Just wait until we get our medals; then we can know who is an old skooler, and maybe even who is a really old skooler.