Something like the following (I'm assuming you have multiple hosts...):
1. Put host in to maintenance mode
2. Remove it from oVirt
3. Change host network settings
4. Re-attach host with new setting to oVirt
5. Migrate some VMs to the new host and fix their IP settings
6. Repeat for other hosts

Things to note:
- If you are using hosted-engine, it will further complicate things for you,
  I'm not sure if and how can one change the H.E. IP.

- If you are using an IP-based storage that is accessed via the same subnet,
  you need to ensure it is still accessible with the same name/IP from the
  new subnet.

If you are only making this change so you can run more VMs, you can
consider adding the new subnet as a new VLAN/physical LAN to the
existing hosts, and keep using old /24 subnet for ovirtmgmt. It is a
good idea to separate ovirtmgmt traffic from other traffic anyway...