Andy is correct, whatever is already on the Target hero is discarded.
Fortunately for me i have a few "play around" accounts on various servers to try stuff on before i try it on my main.
IMHO u8nless you have oodles of coin, and a hero you will never use again with great sancs, and stars, transfer is a pretty useless thing.
Also, though I'm sure y'all'v noticed, you can only transfer within same rank, i.e. you cannot transfer from a blue hero to a purple.