Answer: You can dig into your existing fountain grass to divide it without uprooting the entire plant. Just make sure that each division has some foliage attached to the rootmass. Replant but don't prune anything back; wait until spring to remove the dead blades and tidy up the plants. Aside from digging to divide, I would simply leave the plants alone - the foliage and spikes (stalks) will add winter interest to your garden and you can cut everything down in the spring when new growth begins.
Enjoy your garden!
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