I had the misfortune of living through a rewrite like that. Even though we were able to adapt large portions of the existing code it still took twice as long as

I had the misfortune of living through a rewrite like that. Even though we were able to adapt large portions of the existing code it still took twice as long as expected. The result was so buggy that we took the unprecedented step of mailing out replacement CDs unsolicited to all registered product owners once the bugs were fixed.