I searched FIRST's website but couldn't find an exact answer. The only thing I found was
this.
It's not really the answer you were looking for but it's the only thing I could find about registering new and pre-existing teams. My personal opinion is that 573 wouldn't be called a rookie since that team has competed before. If I do find "concrete" information about this I will post it or put a link to it.
As for losing your team number (and identity) I don't believe you have to worry about that since FIRST doesn't recycle the numbers like they used too.
Jason
(WOW! My 1000th post!

)