There is no option to retain the existing serial number in vSZ.
Was there a specific reason behind retaining the existing serial number? Licensing can be very easily transferred from an old vSZ to a newly built one by logging a case in the Ruckus Support portal:
2. Select case type 'vSCG/vSPOT RMA'.
3. Enter the old/new serial numbers and presuming the vSZ can reach the internet, it will re-sync licensing.