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The old and new fluid will mix a little bit in the reservoir bleeding the brakes. It’s not enough to worry about because the volume of the reservoir is cycled several times over. That said, brake fluid is relatively inexpensive and it bottles can’t be stored long once opened (due to moisture absorption), so there is no downside to suctioning out the old fluid first and using the entire bottle of brake fluid for the service. For what it’s worth, that’s what I do, even though it’s probably not necessary. A turkey baster works well.
Also, I installed SpeedBleeders on all of the calipers to further reduce the chance of air getting back into the system. Again, not necessary, but doesn’t hurt.
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