When I did it, I was wearing an integrated jacket BC (Large) over a tshirt & swimsuit
My budy was in an older hardpack BC (small - the rainbow vest stile with the thick, plastic backplate).
The instructor was in a Zegal ranger (Large)
We got in the pool and the instructor motioned to swap with me so we did, no problem. Even though I had never had his Zegal on before, I was able to figure out the straps and stuff prety quick.
Then he motioned me and my buddy to swap...
Well, I was able to get into the small BC with a little wiggling (don't know if I could have done it on dry land without help...). Only having 1 waist buckle and a keeper on the console it was prety simple.
My buddy was swiming around in that large, strap/flap covered Zegal, occationaly looking out through the openings...
He got it on OK but it took a while to get it straightened out.
Last, we all swapped back at once, round robin stile... There were others there watching us in case anything went wrong.
The instructor wanted to be sure we were comfortable enough to handle unfamiliar, ill fitting equipment and I thought we did prety good.
The mask, snorkle, fin ditch/don was the only hard part.
Swiming down, removing everything and surfacing was easy. going back and putting it all on AND clearing the mask before surfacing was a bit rough.
I found doing the full gear ditch/don with a regulator available to be trivial in comparison.