Schottky diodes for voltage conversion
Those are clamp diodes. They prevent the voltage on the pin from going above Vcc+Vf or below GND-Vf. They do not perform level conversion, and they should not be used to do so since they usually can't take a lot of abuse.

The easiest way to perform level conversion is to use a IC such as TI's TXB01xx chips, but it is possible to use a MOSFET with some pullups for it.