Move dhcp lease to new ip+reservation. How?

[Gregory Sloop]

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I have a subnet in dhcpd - lets just assume 192.168.1.0/24
(It's a fail-over served pool - if that matters.)

I have a pool where unknown-clients are allowed
192.168.1.21-40

I'd like to add a new lease for a machine where the IP is outside the unknown pool above. (I don't want to use a host definition with an IP in the conf files, because I want the ddns name to get added via the DDNS mechanisms - which doesn't happen in that case. Plus, if this machine/device gets moved to another subnet, and the host def is still there, it won't get ANY lease in the new subnet - which is bad. I'd like the device to still function if it gets dropped into a new subnet, even if it's not getting a "special" ip any more.)

This new machine/device may have already been added to the network and currently has an address in the 192.168.21-40 pool.

Lets assume I'd like to assign it 192.168.1.51 - and set a reservation. 
Lets assume that I'll have several machines I'd like set as "static" between 51-70. 
But I don't want just "any" machine to get one of these "special" addresses in the 51-70 range.

What's the best way to go about this?

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Some thoughts I've had, but this gets complicated.

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I don't believe I can just add or modify the lease without changing the pool, because even if there's a defined lease, this is still an unknown client. So, even if there's a reserved lease for 192.168.1.51 - the DHCP server won't give out that address because this is an unknown client. (Right?)

Yet if I make a pool for 51-70 and allow unknown clients, then any client might (will) get one - not just the ones I want to "move" there.

I've thought about pre-creating leases for 51-70 and essentially adding "bogus" information for those leases and reserving them. (While allowing unknown-clients for the 51-70 pool - but since they're all "taken" it won't hand one out),  Then when I want to move something there, I can remove the "bogus" reservation and move the "real" lease to the appropriate IP in the 51-70 block/pool.

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Or define the MAC address in a host definition, without an IP definition. (I think DDNS works in this case.)
Then define the 192.168.51-70 pool as "known" hosts only. (And make sure no "other" known hosts accidentally grab one of the IP's in this pool. This part worries me.)

But it seems like I must be making this too hard.
Am I missing something?

Surely someone else has done this and can point me a tried-and-true solution that works without a ton of drama. :)

(Yes, my pools are larger than those, but the details are essentially the same - this example is just more manageable.)

Thanks!
-Greg