That still wouldn't be safe. The possibility of using "massed Finite Incantatem", as raised by Quirrell, suggests that low-level casters can dispel high-level ones' spells, either by combining power (circumstantial help modifier?) or by getting a natural 20. And matters being as they are, if Voldemort gets transfigured, odds are good that it will be done by Harry (who has a low caster level).