The latest UAG supports Horizon 7.5, and later, so that's not an issue at all, see https://www.vmware.com/resources/compatibility/sim/interop_matrix.php#interop&569=&326=

The required downtime can be minimized to a few minutes, if you configure firewall settings, and deploy the UAG upfront. What will require some downtime in this case is to NAT the external URL/IP to the UAG instead of your security server. External users will be disconnected , but should be able to reconnect through the UAG again.

That said, things may be a bit different depending on your current setup, e.g. number of connection, and security servers, the use of load balancers, etc..

André