What's your actual requirement here?
Why would you want to carry over active leases?
If you swap out your DHCP server, DHCP clients won't be affected immediately.
They will continue to use the IP address of their existing lease until it expires, attempting to renew their existing lease before it does (typically after half their leasetime).
Dynamically allocated DHCP leases are volatile by design, i.e. it is to be expected that -subject to availability- the same MAC may be assigned a different IP address from the pool when requesting or renewing a lease.
If you absolutely must have the same IP being assigned to a MAC, then you'd have to create a DHCP lease reservation/static DHCP lease for that MAC.