There is some limited room to change pins when designing a custom board, but there are restrictions relating to FlexIO and FlexPWM.
In fact it is possible to have both I2S input and output available if you are using T4.1, it just wasn’t possible to do this and keep it compatible with T4.0 (without needing to solder to the bottom pads, that is)…
Any of the pin configurations in this Google Sheets document should work, although I certainly haven’t tested them all. The I2S capability is listed under the column “Audio Shield”.