The way I read the datasheet
"The ADDR pin is used to set the slave address of the MCP23009 (I2C only) to allow up to eight devices on
the bus using only a single pin. Typically, this would require three pins."
you are only setting the I2C address the part responds to, and not changing on the fly to select which GPIO is being accessed - that is via registers.
So setting the voltage once via trimpot and multimeter would be sufficient.
Why not go even simpler, faster, and use the SPI interface instead?