OK, now I've got a dryer with a NEMA 10-30 plug and I'd like to be able to plug it into a NEMA 10-50 receptacle. How much of a risk am I taking if I just change the cord on the 30-amp dryer and plug it into the 50-amp circuit? Thanks.

A dryer needs a 30 amp circuit, you can use a 50 amp rated wire and outlet, but the breaker needs to be a 30 amp.