We would like to comment but we do feel we have enough information. The diet was changed from what to what? Seizures can be the product of an allergy. Has an exclusion diet been tried? Allergies can develop at any time. Also Dog foods reformulate and they are not required to tell you, so there can be an ingredient\s that were not there before. So please let the forum exactly what food your dog is currently being fed and how long they have been on it. This way it can be looked at carefully for anything that might be a trigger.
Its also important to know what treats, brand and type, that you give your dog. Also please list any supplements you might be giving your dog.
Recently we had friends that were giving their dog a supplement to their diet. The dog started having seizures. When I suggested they take away the supplement because it might be a source of an allergy, the seizures went away, and the dog is back to normal.
We are not suggesting that your case is similar, but sometimes its something very simple that you would not expect that causes the issue.