Why switch? Another option would be to list on both .ca and .com. If possible keep stuff that primarily interests Canadians listed on .ca and list things mostly Americans are buyiing on .com.
One problem that can arise when listing on multiple sites is that occassionally customers end up with mixed currencies in their carts which blocks them from paying or requesting a combined invoice. Keeping similar items, similar categories together on one site or the other lessens the likelyhood of cart problems.
There are advantages to listing on .ca versus .com. The three primary ones being more Canadian buyers, lower fees and availibility of the shipping calculator.
When listing on .com in USD slightly higher payment processing fees and currency conversion fees come in to play. Also, most sellers listing on .com cannot use calculated shipping which means offering either free or flat rate shipping. Some use shipping services such as Chit Chat overcome the calculator problem.
