ABI compatibility: unions

Recently we've been adding unions in our public API:

struct di_edid_detailed_timing_def {
	/* Pixel clock */
	int32_t pixel_clock_hz;
	…

	union {
		/* Flags for ANALOG_COMPOSITE signals */
		struct {
			…
		} analog_composite;

		/* Flags for BIPOLAR_ANALOG_COMPOSITE signals */
		struct {
			…
		} bipolar_analog_composite;

		…
	};
};

Is that really all right when it comes to ABI compatibility and future extensibility? What happens if we want to add a new top-level field in struct di_edid_detailed_timing_def? What happens if we add it after the union field, then need to add a new field into the embedded di_edid_detailed_timing_def.analog_composite struct itself?