I want to connect to a PCF8574 I/O expander board. This board is +5V powered with 4k7 pullup resistors to +5V on SDA and SCL. I will common the grounds. Will the Photon be happy with this given its 3V3 environment?
Yes, the digital pins on the photon is 5V tolerant, the analog pins are only 5V tolerant when used as digital pins.
https://docs.particle.io/datasheets/photon-datasheet/#peripherals-and-gpio
All pins except A3 and DAC are 5V tolerant (when not in analog mode). If used as a 5V input the pull-up/pull-down resistor must be disabled.
I understand the I2C pins are 5V tolerant, so believe you should be ok. (Be sure however to disable associated pin input pull up/downs per the datasheet guidance).