I can certainly see this stadium expanded and/or upgraded sometime in the future. I'm quite confident that Toronto can produce way more than 20,000 soccer fans for Toronto FC, given Toronto's experience with the recent World Cup. It appears that there will be plenty of room at Exhibition Place for stadium expansion.
As for the Argos, I don't see them moving to the soccer stadium in the foreseeable future. Their decision to stay at SkyDome appears to be their vote of confidence to the upgrades that Rogers has brought to the stadium. Remember that the renovations to SkyDome are still not complete, so the Argos are probably wanting to stick around to see how much better the place will turn out to be.