ooo it can be quite tricky without rerooting. A doll's part line is made by rooting denser hair at the part section and "thatching" it (left side goes to right and so on)
So if the doll's head is bald, you can do it, you simply have to ignore the original part line and stab new holes into where you want the new part to be. This has advantages, as old part areas can be quite damaged by how closely rooted the hair was, and can tear. I've had to move partings on dolls due to torn scalps before now, it's not fun.
I suggest you look up "doll rooting patterns" for pictures, or just study the existing part line on a balded doll. See how it's like loads of holes all close together? you want to replicate that, but to the side instead and sweep the hair over the old part line (i'd suggest rooting around it, not putting any hair into the old part line at all until you have the rest done, then add bits wherever the hair feels a bit thin. Otherwise you'll end up with a double part and it'll be a mess)