Do I void the contract if I agree to a variation in a schedule ?

I would have though so.
But this site http://www.afcea.org/signal/articles/anm..says
“The first Collins submarine was commissioned inl 1996 with known combat system processing and interface problems. The legacy Oberon CSS MK2 distributed architecture could not handle the heavy contact load that today’s missions require. Two years later, a fast-track interim software fix was begun with some combat [...]