Size matters!The first difference in a proposed C Series 500 would increase this length of fuse by about 10 feet. Currently CS300 is 38m vs. 41m's on a proposed additional plug. The wing span could need to be lengthened as it's currently about 115 ft. Aero testing would need done but this could be accomplished well within a reasonable time frame. Perhaps winglet extensions. As for additional weight assuming 8,000lbs for a new plug. The CS 300 has a max take off weight of 129,000lbs and a base take if weight of 116,000 so adding the 8,000 to the base weight for a new total of 124,000 lbs the existing power plant should be able to easily contend with the job.
Competing against the Airbus A320 and Boeing 738 is the Ace in the whole for longevity for the Bomber. I'm of the opinion this will work a be the Swan Song the company needs to set the record straight. Extending the C Series will happen and delivery will follow success of these new jets soon to come to market in my opinion.