The whole purpose of the Forum is to ask/answer questions. When our founder, Cynthia, was diagnosed she had all sorts of questions and couldn't find good answers...which is why she has devoted much of her life since recover to this web site and Forum.
The answer to your question is no. High grade means that the cells are relatively rapidly dividing and are quite abnormal. Low grade means that there are not as many abnormal cells and they are not as rapidly dividing. With high grade, because of the rapid growth, there is more danger of the cancer spreading, which is why something like BCG is used to stir up the immune system to knock out the cancer cells. IF the cancer spreads, it would first be into the muscle of the bladder and into the lymph nodes around the bladder. And this would not just happen overnight. IF that were to happen the cancerous bladder can be removed but your.dad's case is no where near this point. Your father's cancer is NON-INVASIVE which means it has been found early and is very treatable.
I had a very similar diagnosis and yesterday was my 11th anniversary of no cancer. This should not terrify you!!
Yes, it is serious and needs to be treated, but these treatments are often highly successful.
Sara Anne