If you're just swapping calipers, there probably isn't much air that travels up through the base hydraulic system, so doing an ABS bleed first is probably not going to cause any problems, and saves doing a second base hydraulic bleed.
However, if you have opened the hydraulic system up anywhere else, an ABS bleed may actually introduce air into the ABS pump. In this situation, bleeding the base hydraulic system first removes any air from the lines, then bleeding the ABS won't introduce air, but remove air from the pump instead. Then a final bleed will remove any air removed from the ABS pump.
If the fluid is very old (hydraulic fluid is hygroscopic, absorbs water), it may be a good idea to bleed the base system first so that an ABS bleed will pull in fresh hydraulic fluid, not the old hydraulic fluid that you are trying to remove.
