Can radiation therapy mutate one form of cancer into a more aggressive form?

Yes: In the way you asked the question it can. It depends on the dose. Low dose exposures can allow cells to survive and this could happen. In the treatment with radiation the cancer is treated to levels that are designed to kill the cells and therefore mutation to more aggression is not likely. There are many resistance cancers however and it is more likely the cancer is already aggressive.

Typically, no: Radiation can cause cells to become precancerous, and potentially develop into a cancer ("secondary malignancy.") pre-existing cancers do not generally mutate into more aggressive grades after radiation exposure.