Now is a good time to divide the phlox and renovate your bed. Try to get the plants into the ground quickly so they can root in before the onset of winter. You can certainly add plants in the Spring if you'd like.

I know I'm a rubbish gardener so you may not find as I have that it is impossible to move a plant from a bed with ground elder without getting ground elder with it.

I've spread ground elder into two or three beds that were free of it until I split some plants and moved them.

We have the same problem with couch grass. What I have done in the past is to pot up plants I want to move and grow them on for a season in the pots. That way you can see if you have brought the weeds out with them. It has taken a few re-pottings to get some of the Irises clean enough to go back.
Now is as good a time as any to do it.
You may lose flowering from the Asters though.

Thanks everyone for your very helpful comments I will proceed with the task this weekend weather permitting. I will definitely go along the lines of Owdboggy and grow them in pots for a season until they show they are clean of the dreaded ground elder.

I did a border exactly this way when I moved to this house 20+ years ago.
I lifted the plants and removed as much gd elder as I could find. then potted them in a ji type compost.
the ground was dug and sieved/riddled twice which caught a lot of 'pips' that I had missed.
The following spring I kept a careful eye out and at the sign of a leaf it was dug up. Touch wood it never came back.

now the couch grass has had the same treatment but I am finding new shoots here and there. :o(