old and new sbs co existing

hi all,

I have just installed a new sbs to replace the old sbs 2003.

the old sbs 2003 still has the mailboxes on, I need to know how to extend the period of time the new sbs 2011 and the old sbs 2003 can co exist on the same LAN?

is there a way to extend this period? 21 days have already passed and the old sbs is shutting down every few hours now.

Thanks