Redoing a site with 50k indexed pages without hurting it
 

Hey guys
a friend is relaunching a site with a new code design layout that currently has 50k pages indexed in google (tube site)

If im not mistaken google has something in place where you can put something in the google webmaster tools to let them know of the site change so it doesnt hurt the site too much?

Any ideas?

Not exactly the same scenario, but the advice here should be useful:

support.google.com/webmasters/bin/answer.py?hl=en&answer=83105