Changing DHCP server on Network ramifications?

Solution 1:

Will all leased IPs suddenly change? - In all likelihood your clients will get a different ip address than the one they got from the router the next time they go through the rebinding phase. That, in and of itself, shouldn't cause any problems.

Note that I said rebinding, not renewing. During the renewal phase the DHCP client will attempt to communicate with the DHCP server that granted the lease. Failing to do so (because you've disabled the DHCP service on the router) the clients will then attempt to communicate with any DHCP server during the rebinding phase, during which time the new DHCP server will either allow the DHCP client to continue to use the leased ip address (which is unlikely since the new server will have no information about the existing lease), or the server will deny the request to continue using the leased ip address in which case the client will request a new ip address (this is the likely outcome).