I think this idea is nice, but heres a few suggestions.
:notice: The lower your HP, the higher your chance of transforming gets. Shouldnt be by too much though, because then it could be easily spammed.
:notice: If you were able to transform, little bit of HP should be taken away. Remember, you did transform prematurely/unnaturally.
:notice: Premature transformation shouldnt be as powerful as a regular one. I think it should have 1/2 of the original power, maybe 2/3?
:notice: If you were able to transform, after you die you shouldnt be able to transform by default. If you are not at the right set of PL, the gambling starts all over again. However, maybe your chances should rise a little every time? You did achieve the technique, after all.
About transkilling. If you cant find a safe way to try and risk it, then dont. Its your decision.