If they change the hashed seed, than hashing it again to verify will return a different result than what the server gave you, so you know they cheated.
Most verifiers check to make sure that the legit hash was used.
How do the verifiers check the legit/original hash was used?
The hash is provided by the gambling service. They can just issue you a fake hash and provide you with a fake solution and you will never know right?
Go to google and type sha256 converter and calculator and open one of the links.
Now, go to gambling site and copy your pre-rolled sha256 hash server seed somewhere. When you roll/spin/whatever server seed will be revealed to you.
Now, go to sha256 calculator and copy/paste server seed, you will get sha256 hash for it.
Compare with saved one(pre-rolled one)