An Erika's Oddish has a Pokemon Power Photosynthisis. It changes all attached
energys into grass energys. If I attached a fire energy to Erika's Oddish,
could I use Energy Trans to transfer it to another Pokemon, thus changing it
back into a fire energy?

No.  You can not Energy Trans the energy because Oddish’s Pokemon Power makes the energy card produce Grass Energy, and not become a Grass Energy card.  Venusaur’s Power states that it only moves Grass Energy cards.

Scott