Many IMs have changing tents, but some don't. If they don't, you might have to go to a restroom to change or something, and it will definitely affect your transition times. Depends what your IM goal is.

I'm doing my first IM in sept and I'm willing to lose a few minutes to get into a bib that doesn't make my crotch hurt like hell for 6hrs. Would I do this for a sprint or an Oly, hell no, but a few minutes over 12-14hrs... worth it.

I found that my tri shorts were way more comfortable over the long rides than biking shorts which surprised me. So wearing the tri shorts for the whole race was fine. Many IMs have changing tents, some don't. Even if they do, the clothing change, especially while hot and sweaty will add time to your transition. Only you know what the comfort difference would be for you, and whether or not it's worth the extra time. Personally, I would give the tri shorts a try for some long rides and see what you think.

I wouldn't consider doing anything more than an oly in tri shorts. You'd have to have an iron crotch to do a 1/2 or a full in 'em. If your bike shorts aren't comfortable enough for both the bike and the run, you should look into some different shorts. Sugoi makes about the best apparel out there imho. Desoto 400 mi. shorts are worth the extra $$$ as well. Put 'em on under your wetsuit in the AM and take 'em off after the medal is around your neck. No need to worry about changing.

I race my Ironmans with Jagged tri Shorts which have a removable chamois which only takes a few seconds to remove for the run, on the bike the extra gel padding does help.

I will be doing my first IM in Aug. and definetely plan on getting my ass used to only riding in tri shorts they seem to work better for me. When i get to the run i think i will change into my favorite running shorts for comfort!Good luck with training!:)

I've done both, bike shorts and tri shorts. Prefer the bike shorts, but a well used pair...not a newish pair with a stiff "chamois." Being a minimalist, I don't get into changing outfits during an IM...and I've never had any problems with chaffing or what not on the run. Bib shorts I'd be leery of at an IM...a serious bathroom stop would have you taking off your jersey before you could drop your bloomers and if your wet from rain and/or sweat it would be a bear to get on.
Like everyone suggested though...give both a try on a training day with a long ride followed by a run.

I hope they do b/c i plan on doing some major changing since its going to be such a HOT race!Ugh!

I go tri shorts --I also train in them and find them more comfortable than the more bulky bike shorts. For my first IM I swam/ride in tri shorts and changed into running shorts(it was hot and I just wanted to feel "fresh") For my second I just left the tri shorts on the whole time. This year will be the same. No real need to change, you are just going to get sweaty icky anyway there is no point.
lala2021 there will be a changing tent.

I don't think that bike shorts offer any real relief beyond 3 or 4 hours. At that point, the saddle will break whatever you're wearing and if you're not comfortable on your throne, you're going to pay the price no matter what you're wearing. That said, tri shorts typically allow better movement and better-positioned padding.

I don't even own bike shorts anymore...I found as long as I used my tri-shorts for training, I was easily able to get as comfortable with them as with bike shorts, so when my last pair of bike shorts were retired, I switched to tri-shorts full time. I figure it simplifies these decisions.

You can always wear the bibs on the swim over your tri shorts. when you come into t2 you just take your bibs and go on with the tri shorts. you choose. Rest assured that no one will be pointing and laughing at you. It will be a crowed transition and no one will notice the naked dude.

IMKY does have changing tents.