Cut back first; this makes dividing a lot simpler. Either divide in situ with a sharp spade or a fork, or take it out first and then divide. You can hack; it won't hurt them if they are not too far along in Spring growth. (New growth should be no more than 6" at time of dividing. There's a bit more leeway when simply transplanting.) Water well after replanting, and don't let the roots dry out while dividing. Work quickly but without panic.

Clump size doesn't matter; cut as many as you want. Be sure to remove the original plant's dead center (roots and all). Have the new site prepared before you divide. Keep the divisions moist their first year.

I wish these were perennial for us in zone 5.

Cut it back and then chop out sections with a shovel. In your zone it is dang near a weed and won't need to be treated delicately.

I also wish it was perennial here. I can grow the green ones but not the purple except as an annual.

I live in Florida. I've moved my fountain grass many times. I just dig it up and plant it where I want it. So far so good. It's pretty hardy. Not only that I have had purple fountian grass sprouting from near by planters. It's the local Homedepot variety.

I live in NW Missouri where Purple Fountain Grass is treated as an annual. I dig mine up in the fall, trim it way back, split if necessary and put in pots under fluorescent lighting until spring. Throughout the winter months they send out new shoots and by Spring are ready for another year in the garden. They do well and I put them back in the garden in the spring. Rather than use special "grow light fluorescent tubes" I use a four tube fixture and alternate between "cool" and "warm" tubes putting two of each in the fixture. Works the same as the more expensive grow light tubes for a MUCH more inexpensive cost.