Everyone dies. But delaying that impending sentence is the holy grail. Science, scorcery, and wishful thinking do all in their power to bestow such delays. Be your deeds pure or evil, cheating death is neither easy nor reliable.
Wanderers beware, these gates open not,
Except to those only who the way have bought.
You may purchase the right to pass me by,
By each guessing my riddles. Care to try?
If a creature is transformed into another creature (say, a human into a dog), then that creature will become more and more accustomed to the new form until a point is reached where reversing the spell is impossible. A second transformation is possible, but the creature will have to go through the whole process again.