Go up in the back, come down in the cup.
Used to work 25 years ago for the then revolutionary Rigby and Peller.
So the volume of these cups are the same.
30d, 32c, 34b, 36a. And so on
40a, 38b, 36c, 34d. These cup volumes are the same.
It's just the band around the rib cage that changes.
Yes your boob will fit in a 36a cup for instance, but it will probably ride up at the back if your actual back is 32inches.
So try the 32c.
Up in the cup, down in the back/band.
A 32c might feel very tight to you if you've been in a 36 band. All designs and brands come up differently on different body shapes and sizes.
My best advice is to start with the right feeling band/back and then match it to best fitting cup.
Yes you'll take many into the fitting room but you can start to understand it.