What kind of issues might I face when moving from 400 kHz I2C to 1 MHz I2C?
If you are going to run an I2C bus with mixed speeds, and you have devices on the bus that can only work at 400 KHz and below, then they must be disabled somehow (e.g. powering them down, so their I2C bus leads will be tristated) when running the bus at the 1 MHz speed, since they will not be able to parse the high-speed addresses and may respond incorrectly.