At U of T, EngSci students cans witch out into any engineering program at any point in the year I'm pretty sure. Basically, if you get accepted to EngSci, you can go to school in September, and just say you want to drop out into ___________, and they will switch you out that day, before school even starts pretty much. You could also e-mail the school around the time you get your acceptance, but basically in the end you can switch out into anything you want.
As for UW, you can switch over the summer if you e-mail the faculty of engineering, but there are no guarantees. My friend got accepted to electrical engineering at Waterloo, then asked to be switched to Civil over the summer and she got switched, although there is no guarantee.
MOST first-year engineering programs are similar, but they all differ by 1-2 courses. ECE and SE will generally have harder calc/physics/programming courses, so it'd probably be harder to switch into those. Mech/civ/geo/enve/mgmt/tron have almost identical first years except 1 or 2 courses, which are usually in the second term. For example, civ/mech/geo/chem/enve/mgmt had exactly the same curriculum in 1A, besides the concepts course specific to that field. Second term is also almost the same, with only 1 or 2 courses being different out of 5 or 6.