Can a unidirectional TVS diode leading to ground be replaced by a bidirectional one?
Yes.
BUT:
If your circuit is &quot;hot-plugged&quot;, connected to a long cable, or connected to a poor powersupply; negative transients may occur. In your current setup the TVS diode protects against those. If you replace with a bidirectional those transients may destroy C1, U1 or both.
If you are prototyping, testing, and just want to use what you have at hand then I would say that a unidirectional is fine, as long as you understand that it adds a bit of risk.