My favourite gimmick with suit jacket transformations into band
jackets is to make a plastron out of fabric that matches the jacket
(or close) with a stand collar and belt that close center back, and
eppaulettes to hide the transformation at the shoulder.
It can even be slapped on the actor quickly while he/she faces
upstage by plants in the chorus, and as Harold turns downstage
again, they finish with the closures behind him.
I even have a show using this technique running right now!
imho, having the actor catch a new hat being tossed to him has
much more "Ta-Da!!", than rummaging around with one he's
been wearing all night, and pulling focus to his
hands/waist....and imagine how rumpled it's going to look, even
after a few performances!