I'd like to see some idea of what they mean by big. An order of magnitude in terms of lines of code and the number of engineers they had for the job and how long it all took versus how long the original took to write etc. One of the things that scares me about something like this are all the seemingly illogical bits of code that were added over time and are actually that way for a reason because they fix some edge case or whatever. Hard to see those not getting swept away in a rewrite as well as a whole bunch of new rarely occurring issues added. A complete rewrite strikes me as something that's almost guaranteed to be buggier than what it replaces, at least for awhile.