Migrating DHCP servers

[Peter Rathlev]

We're about to move several networks from being serviced by a Windows
DHCP server to being served by ISC DHCP instead.

What can we expect of trouble if we simply replace the "ip
helper-address" configuration on the router? I tried poking around in
the archives, and stumbled upon this:

https://lists.isc.org/mailman/htdig/dhcp-users/2009-October/010139.html

But it didn't make me significantly wiser.

Two specific questions:

1) Clients that already have a lease would keep asking (unicast) their
current server. Will they do that for as long as the old server keeps
acknowledging their lease, never returning to broadcast again to hear
about the new server? If yes, do I have other options than either
forcing all clients to reboot or disabling the old DHCP server?

2) With "ping-check true;", and assuming all clients will answer to ping
requests, should I have any serious concerns about migrating without
keeping lease state?

If there are any other things I should be worried about I'd love to hear
about them too. :-)

-- 
Peter