Unless you have told your story wrong, it will not work. If the old cord
was 50a, it is the wrong outlet for your new 30a plug. It will not fit.
So, unless you want to replace it yourself (which, given your question, I
would highly recommend against) you will have to have an electrician in. He
will check to make sure the breaker, wire, outlet and dryer are all
compatible.

It is important that no part of the circuit be rated lower than the breaker,
and that the load not be rated higher than the breaker. As long as you do
that they do not have to be exactly the same.