How to change IP address pool in DHCP scope without affecting existing leased IPs

The added net-update command in virsh should allow an dhcp-host update without restarting the virtual network (I have not tested it yet).

man virsh:

net-update network command section xml [--parent-index index] [[--live] [--config] | [--current]]

Update the given section of an existing network definition, with the changes optionally taking effect immediately, without needing to destroy and re-start the network.

command is one of "add-first", "add-last", "add" (a synonym for add-last), "delete", or "modify".

section is one of "bridge", "domain", "ip", "ip-dhcp-host", "ip-dhcp-range", "forward", "forward-interface", "forward-pf", "portgroup", "dns-host", "dns-txt", or "dns-srv", each section being named by a concatenation of the xml element hierarchy leading to the element being changed. For example, "ip-dhcp-host" will change a <host> element that is contained inside a <dhcp> element inside an <ip> element of the network.

xml is either the text of a complete xml element of the type being changed (e.g. "<host mac="00:11:22:33:44:55' ip='1.2.3.4'/>", or the name of a file that contains a complete xml element. Disambiguation is done by looking at the first character of the provided text - if the first character is "<", it is xml text, if the first character is not "<", it is the name of a file that contains the xml text to be used.

The --parent-index option is used to specify which of several parent elements the requested element is in (0-based). For example, a dhcp <host> element could be in any one of multiple <ip> elements in the network; if a parent-index isn't provided, the "most appropriate" <ip> element will be selected (usually the only one that already has a <dhcp> element), but if --parent-index is given, that particular instance of <ip> will get the modification.

If --live is specified, affect a running network. If --config is specified, affect the next startup of a persistent network. If --current is specified, affect the current network state. Both --live and --config flags may be given, but --current is exclusive. Not specifying any flag is the same as specifying --current.

Why don't you set the IP for that appliance explicitly:

# The standard subnet
subnet 10.0.0.0 netmask 255.255.255.0 {
    option domain-name "ourdomain.internal";
    option routers 10.0.0.1;
    option domain-name-servers 10.0.0.2;
    range 10.0.0.10 10.0.0.49;
    range 10.0.0.51 10.0.0.246;
}


#has hardcoded ip, and dhcp should not use that in pool
host fixedipappliance {
    hardware ethernet 10:4e:ed:co:ff:ee;
    fixed-address 10.0.0.50;
}

I just successfully tested a configuration with two ranges like that. Multiple range statements are legal, as per the manpage there must be at least one range statement:

The range statement

  range [ dynamic-bootp ] low-address [ high-address];

For any subnet on which addresses will be assigned dynamically, there must be at least one range statement. The range statement gives the lowest and highest IP addresses in a range. All IP addresses in the range should be in the subnet in which the range statement is declared. The dynamic-bootp flag may be specified if addresses in the specified range may be dynamically assigned to BOOTP clients as well as DHCP clients. When specifying a single address, high-address can be omit‐ ted.