Yes, you can use the same license both on the MR18s and the MR36s. If you are using the Co-termination licensing model you won't have to move licenses from old devices to new devices, you will simply have to remove the unused MRs from the networks they are assigned to.
If you are using the Per-device licensing model, you will have to unassign the license from the old device and reassign to the new device.
BR,
Ioana