I've done it at justbet, bol, and betus. Not a regular thing. But sometimes you make a mistake. All three have been pretty cool about it. Usually you get the "one time exception" spiel but that's fine.I did this recently at Justbet in September, and they easily with no problem canceled by bet no questions asked.
Lame that Heritage wouldn't take care of a loyal customer.
Out of curiosity, did the line move? That would be a legit reason not to cancel it.