Do we need a new license or do we do something with the old license?
Generally speaking, you do not need a new license if you have an existing license that is available to use.
If you have been running RPM on a machine for some time, and the support was allowed to lapse, you can reinstate support on that license without affecting the legitimate use of that license. Reinstating support has literally no effect on RPM assuming the license was already active, and not in some other state like trial mode (which is not from support lapsing).
If your support has lapsed a very long time, it may be less expensive to purchase a new license than to reinstate support. Our technicians can help you in that circumstance.
We try to keep the licensing terms as straightforward as possible. You can use as many RPM licenses as you have purchased, and you can certainly use all of them at any given time.