Additional Study Information: You are being asked to participate in this study because you have a myeloproliferative neoplasm (MPN) that has transformed to accelerated or blast phase. MPN is a group of diseases in which the bone marrow makes too many blood cells. The different types of MPN are myelofibrosis (MF), essential thrombocythemia (ET), or polycythemia vera (PV). Over time, these diseases may become harder to treat by entering an accelerated or blast phase. These phases occur when the bone marrow makes too many immature cells called blasts. Blasts are not fully developed and cannot carry out any particular function in the body. Accelerated phase is when there are 10 to 19% blasts in the blood or bone marrow. Blast phase occurs when there are more than 20% blasts in the blood or bone marrow. Fedratinib is a drug approved for myelofibrosis that blocks the Jak proteins that are often abnormal in MPNs. Decitabine is an approved chemotherapy drug for myelodysplastic syndrome and acute leukemia, which have too many blasts similar to advanced MPNs. Recent studies have shown that ruxolitinib, another drug that blocks Jak proteins, could put some patients with your disease into remission (a decrease in or disappearance of signs and symptoms of your disease) and that ruxolitinib is more effective together with decitabine. Because many patients with MPNs have taken ruxolitinib by the time their disease worsens, we are using fedratinib in hopes that it may be more effective. Decitabine is given daily by vein for 5 days and repeated every 4-6 weeks. Fedratinib is an oral medication taken daily.