If I were taking part this year and the weather was as autumnal as last year, I would go for a full change (at least of top half) in T1 just to get dry clothes on and be warmer. Probably a jersey and arm warmers with a jacket in my pocket. I'd probably include another top for the run and include 3/4 length tights in case I was cold. If I wasn't cold/wet coming off the bike, I may not use any of it.
This is basically my plan 2 x full changes - cycle shorts, cycle top and arm warmers,(without the 3/4 length tights), although I only have a very light shower proof in cycling top. into run shorts and tri top. I have considered putting on a base layer in T1 as I was a bit chilly on this weeks long ride.