Well if you want to split the flow you always can. The existing cases might not be impacted depending on your development approach. If you decide to work with different flow names you should be fine, say for example your existing flow is called "Approval0", then if for your new development you work with "Approval1", "Approval2" .........you will be fine, the existing cases on "Approval0" will never call the new ones.
Now of course you may want to keep the same flow name, in that case, the second approach would be to "circumstance" the flow with the same name. With the circumstance in place, only the brand new cases will using the new flow, the old cases will be using the previous version.