Let's have a quick look in the 130 seater market,
as this one is clear defined - before the CS300 was launched there was only the B737-700 and the A319 avaible.
I looked at the backlog as it was in July 2008, as it is today (July 31st. 2011) and how many aircraft were delivered in between.
I counted only the aircraft ordered and delivered to airlines, not to private customers.
One can find from that numbers that there have been 232 cancellations for the B737-700 and 208 cancellations for the A319 during that time. Most of these cancellations were "just" conversions to the B737-800 or -900 or to the A320/A321.