What does it mean when you go from a 36C to a 36D in bra size?

Well, darling, going from a 36C to a 36D means your cup size has increased by one size. So, basically, your girls have decided to upgrade to a roomier accommodation. It's like moving from a cozy studio apartment to a slightly bigger one - more space, more support, and maybe a little extra va-va-voom!