Depends.
First, it depends upon the booking code. More precisely, it depends upon the AC booking code that the United booking code translates into. At the bottom end, AC charges separately for seat assignments. also, people with no seat assigned are the highest priority for involuntary denied boarding, i.e. bumped.
Plus, in order to get a seat assigned, at least, to do it on aircanada.com, you will need the AC booking refernce (locator), which differs from the UA one. If your TA is not too clueless, he might be able to find it.
Otherwise, you would need to call AC; you might after an hour waiting on the phone for an agent, end up talking to someone willing to help. Or not.