Anything already growing should be unaffected by a preemergent herbicide. Preemergents stop a seed from taking root.

@dchall I was thinking the same thing when I read this post but then I got to wondering. It is a good question. If the premerg allows the seed to germinate but stunts the growth of the root, then can it possibly retard the growth of new roots of something like KBG that would otherwise spread into a bare area?

If I had to take a guess I'd say it is not a problem but because I don't know exactly what the premerg does to the seed it is hard to conclude. I assume the OP searched the manufacturer's website for info like that.