For FE8U:
At 0x33750 and 0x9543E, the game branches to 0x953F0 (which eventually branches to 0x84800 to check the current chapter’s forced units list). The branch done at 0x9543E seems to be for checking if a starting position should show up as blue (an allowed starting position), while the branch done at 0x33750 is for checking if the unit that is swapping (or is being swapped with) can change their position.
Writing 0x00 0x20 0x00 0x00 at 0x33750 and 0x9543E will allow forced units to swap with other deployed player units, and also allow other player units to swap to those forced unit positions.