I can't speak to the intention of the government when it announced that in 2011. I can probably safely say that there are two main reasons for doing that. One reason can be to reduce the overall cost of the program. The other reason can be to provide further work incentives. When the country is faced with labour shortages, it provides an incentive for older persons to remain in the workforce for a couple more months or up to two more years.
The impact this would have had on seniors is difficult to determine precisely. It's quite clear that in the absence of old age security for an additional two years, there would be more seniors living in poverty.
I should point out, however, that the change was scheduled to be implemented very gradually over a number of years. I don't remember exactly the timeline over which it was to be implemented.