EZH2 is frequently overexpressed in glioblastoma (GBM), suggesting an oncogenic function that could be a target for therapeutic intervention. However, reduced EZH2 activity can also promote tumorigenesis, leading to concerns about the use of EZH2 inhibitors. Here, we provide further insight about the effects of prolonged Ezh2 inhibition in glioblastoma using preclinical mouse models and primary tumor-derived human GBM cell lines. Using doxycycline-inducible shRNAs that mimic the effects of a selective EZH2 inhibitor, we demonstrate that prolonged Ezh2 depletion causes a robust switch in cell fate, including significantly enhanced proliferation, DNA damage repair, and activation of part of the pluripotency network, resulting in altered tumor cell identity and tumor progression. Short-term Ezh2 depletion significantly improved survival without the tumor progression observed upon prolonged Ezh2 depletion, suggesting that precise dosing regiments are very important. These results could be of high clinical relevance with regard to how glioblastomas should be treated with epigenetic therapies.